BRAF (v-raf murine sarcoma viral oncogene homolog B) and MEK (mitogen-activated extracellular signal-regulated kinase) inhibitors are the cornerstones of targeted therapy for BRAF V600E (valine-to-glutamate at codon 600)-mutant melanoma, while CDK4/6 (cyclin-dependent kinase 4 and 6) inhibitors have transformed the treatment of hormone receptor-positive, HER2 (human epidermal growth factor receptor 2)-negative breast cancer. Both drug classes carry distinct pharmacogenomic requirements and toxicity profiles that demand individualized management.
BRAF V600E and the MAPK Pathway. The MAPK (mitogen-activated protein kinase) pathway, operating through the sequential activation of RAS (rat sarcoma viral proto-oncogene), RAF, MEK, and ERK (extracellular signal-regulated kinase), is one of the most commonly mutated oncogenic cascades across human cancers.1 The BRAF V600E (valine-to-glutamate substitution at codon 600) mutation constitutively activates the BRAF kinase domain, bypassing the need for upstream RAS activation and driving continuous ERK signaling that promotes proliferation and survival. BRAF V600E occurs in approximately 50% of cutaneous melanomas, 60% of papillary thyroid carcinomas, 10% of colorectal cancers, and smaller proportions of lung adenocarcinomas, hairy cell leukemia, and gliomas. BRAF V600E in colorectal cancer predicts poor prognosis and resistance to BRAF inhibitor monotherapy (explained by feedback EGFR reactivation), requiring a distinct combination strategy.
BRAF Inhibitors: Vemurafenib and Dabrafenib. Vemurafenib and dabrafenib are selective inhibitors of mutant BRAF V600E that produce rapid, high-magnitude responses in BRAF-mutant melanoma; response rates of 48-53% and progression-free survival of approximately 5-6 months were established in pivotal trials, representing dramatic improvements over prior chemotherapy.1 Both agents are orally administered and metabolized predominantly by CYP3A4 (cytochrome P450 3A4), making them susceptible to induction (loss of efficacy with rifampin, carbamazepine, St. John’s wort) and inhibition (elevated exposure with azole antifungals, clarithromycin). Vemurafenib also inhibits CYP1A2 (cytochrome P450 1A2) and is an inducer of CYP3A4 at higher concentrations, creating complex drug interaction profiles. Dabrafenib is metabolized to active metabolites by CYP2C8 (cytochrome P450 2C8) in addition to CYP3A4. A clinically important and unique toxicity of dabrafenib is pyrexia (fever), which occurs in approximately 28% of patients, is dose-dependent, and requires interruption and corticosteroid use in severe cases; the mechanism involves inhibition of BRAF-mediated pathways involved in temperature regulation.
Paradoxical MAPK Activation: The Critical Safety Concept. When BRAF inhibitors are used as monotherapy in patients with RAS-mutant tumors (e.g., KRAS [Kirsten rat sarcoma viral proto-oncogene]-mutant colorectal cancer), they paradoxically activate ERK signaling rather than suppress it.2 This occurs because BRAF inhibitors, by occupying one protomer of the RAF dimer, promote transactivation of the unoccupied protomer (CRAF or the second BRAF), which then drives MEK (mitogen-activated extracellular signal-regulated kinase)/ERK activation. This paradoxical activation is the mechanistic basis for two critical clinical consequences: (1) BRAF inhibitor monotherapy is ineffective and potentially harmful in RAS-mutant cancers, and (2) BRAF inhibitor therapy accelerates growth of pre-existing RAS-mutant lesions, manifesting clinically as cutaneous squamous cell carcinomas (cSCCs) and keratoacanthomas in approximately 15-30% of patients treated with BRAF monotherapy. The combination of a BRAF inhibitor with a MEK inhibitor suppresses paradoxical ERK activation, reduces cSCC incidence, and is the required standard of care.
MEK Inhibitors: Trametinib and Cobimetinib. Trametinib and cobimetinib are non-competitive, allosteric inhibitors of MEK1/2 (MEK isoforms 1 and 2) that suppress ERK phosphorylation downstream of all RAF isoforms, regardless of RAS mutation status.1 Trametinib is primarily metabolized by deacetylation and glucuronidation with minor CYP3A4 involvement; cobimetinib is a CYP3A4 substrate. The BRAF-plus-MEK inhibitor combinations of dabrafenib-trametinib and vemurafenib-cobimetinib demonstrated significantly superior progression-free survival compared to BRAF inhibitor monotherapy in BRAF V600E-mutant melanoma (the Combi-d/v trials [randomized studies of combined dabrafenib-trametinib] for dabrafenib-trametinib; coBRIM trial for vemurafenib-cobimetinib), and both combinations reduced cSCC incidence from approximately 19-24% to less than 1-2%. Additional approved combinations include encorafenib-binimetinib (COLUMBUS trial). For BRAF V600E-mutant colorectal cancer, the combination of encorafenib with cetuximab (an anti-EGFR antibody) is the standard approach, since EGFR pathway suppression blocks the adaptive feedback reactivation that limits single-agent BRAF inhibitor efficacy in this tumor type.
BRAF (v-raf murine sarcoma viral oncogene homolog B)/MEK Inhibitor Toxicity Profile. The combination of BRAF and MEK inhibitors produces a composite toxicity profile reflecting both on-target and off-target effects of each agent.2 Dermatologic toxicity remains prominent but shifts character: while cSCC and keratoacanthoma are markedly reduced by MEK inhibitor co-administration, hyperkeratosis, hand-foot skin reaction, rash, and alopecia persist. Ocular toxicity is a class effect of MEK inhibitors, manifest as serous retinal detachment (chorioretinopathy), retinal vein occlusion, and blurred vision; baseline ophthalmologic evaluation and periodic monitoring are required. MEK inhibitors also cause cardiomyopathy with LVEF (left ventricular ejection fraction) reduction, occurring in approximately 7-11% of patients on trametinib, requiring baseline echocardiography and monitoring every 8-12 weeks during treatment. Pyrexia, prominent with dabrafenib, is managed by dose interruption and brief corticosteroid courses. QTc prolongation can occur with vemurafenib. Photosensitivity is a class effect of vemurafenib requiring strict sun protection. Liver function test (LFT) elevation occurs with both classes and warrants monthly monitoring for the first 6 months.
BRAF inhibitors (vemurafenib, dabrafenib) must never be used as monotherapy in RAS-mutant tumors (KRAS, NRAS, HRAS mutations). Paradoxical MAPK activation accelerates tumor growth and produces no clinical benefit. Always verify BRAF V600E mutation status before initiating therapy. In colorectal cancer with BRAF V600E, use encorafenib with cetuximab, not BRAF inhibitor monotherapy. In any cancer with concurrent RAS mutation, standard BRAF/MEK combination therapy is also unlikely to be effective and should not be used outside clinical trials.
CDK4/6 Inhibitors: Palbociclib, Ribociclib, and Abemaciclib. CDK4 (cyclin-dependent kinase 4) and CDK6 (cyclin-dependent kinase 6) are serine-threonine kinases that, when complexed with D-type cyclins, phosphorylate the retinoblastoma (Rb) protein, releasing the E2F (E2 factor) transcription factors that drive S-phase entry and cell cycle progression.3 In ER (estrogen receptor)-positive, HER2 (human epidermal growth factor receptor 2)-negative breast cancer, cyclin D1 (a cell-cycle regulatory protein encoded by the CCND1 [cyclin D1] gene) is frequently overexpressed due to estrogen-driven transcription making CDK4/6 a rational therapeutic target. The CDK4/6 inhibitors palbociclib, ribociclib, and abemaciclib all bind the CDK4/6 ATP (adenosine triphosphate)-binding pocket as competitive inhibitors, preventing Rb phosphorylation and arresting cells in G1 (gap 1) phase. All three are approved in combination with aromatase inhibitors (for postmenopausal or ovarian-suppressed premenopausal patients) or fulvestrant (for endocrine-refractory disease), based on trials demonstrating significantly superior progression-free survival compared to endocrine therapy alone: PALOMA-2 (palbociclib), MONALEESA-2/3/7 (ribociclib), and MONARCH-2/3 (abemaciclib).
CDK4/6 Inhibitor ADME (Absorption, Distribution, Metabolism, Excretion) and Drug Interactions. All three CDK4/6 inhibitors are orally administered and primarily metabolized by CYP3A4, making them susceptible to the same class of CYP3A4-based drug interactions as many other targeted agents.3 Palbociclib has oral bioavailability of approximately 46% with food; it should be taken with food to maximize absorption. Ribociclib is a moderate CYP3A4 inhibitor in addition to being a substrate; it also prolongs the QTc interval, requiring a baseline ECG (electrocardiogram) and repeat ECGs at day 14 of cycle 1 and the start of cycle 2, with the drug withheld for QTc exceeding 480 ms. Abemaciclib has the highest oral bioavailability (approximately 45%) and is distinguished by more continuous dosing (twice daily rather than 3 weeks on, 1 week off for palbociclib and ribociclib) and by activity as a single agent (approved monotherapy in heavily pretreated ER+/HER2- breast cancer). Strong CYP3A4 inhibitors (azole antifungals, clarithromycin) increase CDK4/6 inhibitor exposure; dose reduction is required with these combinations. Strong CYP3A4 inducers (rifampin, carbamazepine) substantially reduce exposure and should be avoided.
CDK4/6 Inhibitor Toxicity and Monitoring. The dominant shared toxicity of CDK4/6 inhibitors is neutropenia, which is on-target (CDK4/6 regulate hematopoietic progenitor proliferation) and typically non-febrile, reflecting cytostasis rather than bone marrow aplasia; it resolves within 1-2 weeks of dose interruption and does not require growth factor support in most cases.3 Palbociclib and ribociclib use a 3-weeks-on, 1-week-off schedule to allow neutrophil recovery. CBC (complete blood count) monitoring is required before each cycle and at day 14 of the first two cycles. Abemaciclib causes substantially more diarrhea than the other two agents (approximately 80%, grade 3 in approximately 10%), driven by CDK4/6 inhibition in intestinal crypt cells; early loperamide and dose reduction for persistent grade 2 or higher diarrhea are standard. Ribociclib causes hepatotoxicity (LFT elevation) requiring baseline and periodic liver function monitoring; it prolongs QTc, mandating avoidance of concurrent QTc-prolonging drugs (antiarrhythmics, antipsychotics, fluoroquinolones, azithromycin). Venous thromboembolism (VTE) risk is increased with all CDK4/6 inhibitors, likely due to tumor microenvironment effects rather than direct drug effects; VTE prophylaxis should be considered in high-risk patients.
The PI3K (phosphoinositide 3-kinase)/AKT (protein kinase B)/mTOR (mechanistic target of rapamycin) pathway is one of the most frequently activated oncogenic cascades in human cancer, governing cell survival, growth, metabolism, and protein translation. BTK (Bruton’s tyrosine kinase) inhibitors have revolutionized the treatment of B-cell malignancies by exploiting chronic B-cell receptor signaling as a vulnerability. Both drug classes generate distinctive toxicity profiles with significant management implications for the practicing clinician.
PI3K Inhibitors: Idelalisib and Alpelisib. PI3K exists as multiple isoforms; the clinically targeted isoforms are PI3K-delta (PI3Kδ) and PI3K-alpha (PI3Kα). Idelalisib is a selective PI3Kδ inhibitor approved for relapsed chronic lymphocytic leukemia (CLL), follicular lymphoma, and small lymphocytic lymphoma; PI3Kδ is the dominant PI3K isoform in B lymphocytes and is required for B-cell receptor and chemokine signaling that sustains malignant B-cell survival and homing.4 Idelalisib has significant immune-mediated toxicities: hepatotoxicity (ALT/AST [alanine/aspartate aminotransferase] elevation grade 3 or higher in approximately 20-40% of patients) requiring monthly LFT (liver function test) monitoring and drug interruption for elevations; colitis (immune-mediated enterocolitis resembling inflammatory bowel disease, occurring in approximately 14% of patients) requiring corticosteroid management and drug discontinuation; and an excess of opportunistic infections including PJP (Pneumocystis jirovecii pneumonia), CMV (cytomegalovirus) reactivation, and invasive fungal infections. PJP prophylaxis (trimethoprim-sulfamethoxazole or alternatives) is mandatory during idelalisib therapy. Idelalisib is a strong CYP3A4 (cytochrome P450 3A4) inhibitor, substantially increasing exposure of co-administered CYP3A4 substrates.
Alpelisib: PI3K-Alpha Inhibitor. Alpelisib is a selective PI3Kα inhibitor approved for PIK3CA (phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha)-mutant, ER-positive, HER2 (human epidermal growth factor receptor 2)-negative breast cancer in combination with fulvestrant following endocrine therapy progression.4 Companion diagnostic testing with the therascreen PIK3CA assay (tissue) or Guardant360 plasma assay is required before prescribing to confirm PIK3CA mutation. The defining toxicity of alpelisib is hyperglycemia, occurring in approximately 64% of patients (grade 3 or higher in approximately 37%), reflecting PI3Kα’s central role in insulin-mediated glucose uptake in adipose tissue, liver, and muscle; insulin resistance, rather than impaired insulin secretion, is the underlying mechanism. Fasting blood glucose monitoring before each cycle and clinical hyperglycemia management (dietary modification, metformin, insulin as needed) is standard. Patients with pre-existing diabetes require particularly close glucose monitoring and dose interruption for blood glucose exceeding 500 mg/dL. Severe cutaneous reactions including Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) have been reported; patients should be counseled to report skin changes immediately. Diarrhea is also prominent (approximately 58%).
mTOR Inhibitors: Everolimus and Temsirolimus. Everolimus and temsirolimus are rapamycin analogs (rapalogs) that inhibit mTORC1 (mechanistic target of rapamycin complex 1) by binding FKBP12 (FK506-binding protein 12), which then allosterically inhibits the mTOR kinase domain.5 mTORC1 inhibition reduces protein synthesis (via S6K1 inhibition), induces autophagy, and suppresses HIF-1 (hypoxia-inducible factor 1)α-driven VEGF (vascular endothelial growth factor) production, contributing to both cytostatic and anti-angiogenic effects. Everolimus is orally bioavailable (approximately 20% fasted, substantially increased by high-fat meal), extensively metabolized by CYP3A4 and P-gp (P-glycoprotein), with a half-life of approximately 30 hours; it is approved for ER+/HER2- breast cancer (with exemestane, BOLERO-2 trial), renal cell carcinoma, pancreatic neuroendocrine tumors, and tuberous sclerosis complex. Temsirolimus is given intravenously and is a prodrug rapidly converted to sirolimus. The dominant clinical toxicities of mTOR inhibitors are stomatitis (oral mucositis, occurring in approximately 40-60%; treat with corticosteroid mouthwash, not antifungals), non-infectious pneumonitis (occurring in approximately 10-14%; presents as bilateral ground-glass opacities, managed by dose interruption and corticosteroids if symptomatic), hyperglycemia (similar mechanism to alpelisib but less severe), hypertriglyceridemia and hypercholesterolemia, and immunosuppression with increased infection risk. Strong CYP3A4 inhibitors increase everolimus exposure and should prompt dose reduction; strong inducers reduce exposure substantially.
BTK Inhibitors: Ibrutinib, Acalabrutinib, and Zanubrutinib. BTK is a tyrosine kinase in the B-cell receptor (BCR) signaling cascade, downstream of PI3Kδ and upstream of nuclear factor kappa-B (abbreviated NF-κB), the master transcriptional regulator of B-cell survival and PLCgamma2, driving B-cell survival, proliferation, and homing.6 Ibrutinib (first-generation) is an irreversible, covalent inhibitor of BTK at cysteine 481 in the ATP (adenosine triphosphate)-binding domain; it also inhibits multiple off-target kinases including TEC (tyrosine kinase expressed in hepatocellular carcinoma), EGFR (epidermal growth factor receptor), ITK (interleukin-2-inducible T-cell kinase), and SRC (proto-oncogene tyrosine-protein kinase)-family kinases, which accounts for several of its unique toxicities. Acalabrutinib and zanubrutinib are second-generation BTK inhibitors with greater selectivity for BTK over off-target kinases, resulting in improved tolerability. All three are approved for CLL (chronic lymphocytic leukemia), MCL (mantle cell lymphoma), WM (Waldenström macroglobulinemia), and marginal zone lymphoma, with specific indications varying by agent and line of therapy.
BTK Inhibitor ADME (Absorption, Distribution, Metabolism, Excretion) and Drug Interactions. All BTK inhibitors are orally administered and extensively metabolized by CYP3A4, making them highly susceptible to CYP3A4-based interactions.6 Ibrutinib has low oral bioavailability (approximately 3-4% fasted; food increases bioavailability approximately 2-fold and should be taken consistently with or without food), a short half-life of approximately 4-6 hours, and generates an active metabolite PCI-45227 (an ibrutinib hydroxylation product with comparable BTK inhibitory activity). Acalabrutinib bioavailability is approximately 25% and is reduced by approximately 50% by PPIs (proton pump inhibitors) due to a pH-dependent solubility requirement; H2RAs (histamine-2 receptor antagonists) taken 2 hours before acalabrutinib are preferred over PPIs. Zanubrutinib has higher oral bioavailability (approximately 60%) and is less affected by gastric pH. Strong CYP3A4 inhibitors substantially increase BTK inhibitor exposure (dose reduction required or alternative agent preferred); strong inducers reduce exposure below therapeutic levels and should be avoided.
BTK Inhibitor Toxicity: Atrial Fibrillation, Bleeding, and Hypertension. The most clinically significant unique toxicities of BTK inhibitors are atrial fibrillation (AF), bleeding, and hypertension.6 AF occurs in approximately 6-16% of patients on ibrutinib (higher than acalabrutinib or zanubrutinib) and is thought to result from off-target inhibition of ITK (interleukin-2-inducible T-cell kinase) and CSK (C-terminal Src kinase) kinases, which regulate atrial cardiomyocyte calcium handling; the mechanism is distinct from typical cardiotoxicity and does not involve structural cardiomyopathy. Management of new-onset AF during BTK inhibitor therapy is complex: ibrutinib significantly interacts with warfarin (CYP3A4 inhibition elevates INR) and with direct oral anticoagulants (DOACs; ibrutinib inhibits P-gp, increasing DOAC levels); moreover, ibrutinib itself causes platelet dysfunction through BTK and TEC kinase inhibition in platelets, creating additive bleeding risk when combined with anticoagulants. If anticoagulation is required, acalabrutinib or zanubrutinib are preferred over ibrutinib due to lower AF incidence and less platelet dysfunction. Hypertension occurs in approximately 22-29% of ibrutinib-treated patients and is treated with standard antihypertensive agents, though avoidance of diltiazem and verapamil (CYP3A4 inhibitors) is recommended. Perioperative management requires holding ibrutinib for at least 3-7 days before major surgery given the platelet dysfunction risk; acalabrutinib or zanubrutinib carry less perioperative bleeding risk.
Patients with CLL or MCL on ibrutinib who develop AF requiring anticoagulation represent a high-risk scenario. Ibrutinib inhibits CYP3A4 (elevating warfarin levels) and P-gp (increasing DOAC levels), while simultaneously causing platelet dysfunction. If anticoagulation is mandated, a DOAC is generally preferred over warfarin, with dose reduction considered given P-gp inhibition. Switching from ibrutinib to acalabrutinib or zanubrutinib should be considered to reduce AF incidence and platelet dysfunction going forward. Aspirin should be avoided in combination with BTK inhibitors due to additive bleeding risk.
BCL-2 (B-cell lymphoma 2), PARP (poly-ADP-ribose polymerase), FLT3 (FMS [FMS proto-oncogene, receptor tyrosine kinase]-like tyrosine kinase 3), and IDH (isocitrate dehydrogenase) inhibitors each exploit specific oncogenic vulnerabilities defined by molecular testing. This section covers the mechanistic basis, pharmacogenomic companion diagnostics, ADME (absorption, distribution, metabolism, excretion), and the unique clinical emergencies associated with each drug class.
Venetoclax: BCL-2 Inhibitor Mechanism and Tumor Lysis Syndrome. BCL-2 is an anti-apoptotic protein that sequesters pro-apoptotic proteins (BAX, BAK) on the outer mitochondrial membrane, preventing the release of cytochrome c and apoptosis initiation; its overexpression is a hallmark of CLL (chronic lymphocytic leukemia) and many other lymphoid malignancies and is the mechanism by which these cells resist apoptotic stimuli.7 Venetoclax is a selective, potent inhibitor of BCL-2 (BH3 mimetic), displacing pro-apoptotic proteins and triggering rapid, synchronous apoptosis of BCL-2-dependent tumor cells. This mechanism of action produces the defining clinical challenge: tumor lysis syndrome (TLS), which can be life-threatening when a large burden of BCL-2-dependent tumor cells undergoes rapid simultaneous lysis. TLS manifests as hyperuricemia, hyperkalemia, hyperphosphatemia, and hypocalcemia with resultant acute kidney injury (AKI), cardiac arrhythmia, and seizures; the metabolic crisis can occur within hours of the first dose. The standard approach is a mandatory dose ramp-up schedule: starting at 20 mg/day in week 1, increasing to 50, 100, 200, and then 400 mg/day over 5 weeks with TLS prophylaxis (allopurinol or rasburicase), aggressive hydration, and laboratory monitoring at 6-8 hours after each dose increase.
Venetoclax ADME and Drug Interactions. Venetoclax has oral bioavailability of approximately 50-65% with a low-fat meal; a high-fat meal further increases exposure and venetoclax should be taken consistently with a meal.7 It is primarily metabolized by CYP3A4 (cytochrome P450 3A4) and is also a P-gp (P-glycoprotein) substrate. Strong CYP3A4 inhibitors dramatically increase venetoclax exposure: ketoconazole increases venetoclax AUC (area under the concentration-time curve) by approximately 6-fold and clarithromycin by approximately 5-fold; both are contraindicated during the ramp-up phase when TLS risk is highest. Moderate CYP3A4 inhibitors (fluconazole, diltiazem, erythromycin) require venetoclax dose reduction. Strong CYP3A4 inducers (rifampin, carbamazepine) reduce venetoclax exposure and should be avoided. Venetoclax is approved in combination with obinutuzumab for CLL (MURANO trial) and with azacitidine or low-dose cytarabine for newly diagnosed AML (acute myeloid leukemia) in patients ineligible for intensive chemotherapy. The combination with azole antifungals, which are frequently required for AML patients, creates a major drug interaction challenge requiring venetoclax dose reduction to 70 mg when posaconazole (a strong CYP3A4 inhibitor) is co-administered.
PARP Inhibitors: Mechanism of Synthetic Lethality. PARP1 (poly-ADP-ribose polymerase 1) and PARP2 (poly-ADP-ribose polymerase 2) are nuclear enzymes that detect and initiate repair of single-strand DNA (deoxyribonucleic acid) breaks (SSBs) through the base excision repair (BER) pathway; when PARP is inhibited, SSBs accumulate and are converted to double-strand breaks (DSBs) during DNA replication.8 In cells with intact homologous recombination (HR) repair, DSBs are accurately repaired by BRCA1/2 (breast cancer susceptibility gene 1/2)-dependent mechanisms. In cells with deficient HR repair (due to germline or somatic BRCA1 (breast cancer susceptibility gene 1) or BRCA2 (breast cancer susceptibility gene 2) mutations, or other HRD [homologous recombination deficiency] alterations), PARP inhibition leads to irreparable DSB (double-strand break) accumulation and cell death. This concept of synthetic lethality, where two individually tolerated molecular deficiencies (PARP inhibition + HR deficiency) combine to produce cell death, is the pharmacologic rationale for PARP inhibitor therapy. PARP inhibitors are approved in BRCA1/2-mutant breast cancer (adjuvant and metastatic), ovarian cancer (first-line maintenance after platinum response and in BRCA (breast cancer susceptibility gene)-mutant or HRD-positive tumors), BRCA-mutant prostate cancer, and BRCA-mutant pancreatic cancer.
PARP Inhibitor ADME, Toxicity, and Clinical Distinctions. The approved PARP inhibitors olaparib, rucaparib, and niraparib share a common mechanism but differ in their selectivity, ADME, companion diagnostic requirements, and toxicity profiles.8 Olaparib is the broadest-approved agent; oral bioavailability is approximately 77%, metabolized primarily by CYP3A4; a high-fat meal increases its AUC by approximately 21%. Niraparib has oral bioavailability of approximately 73%, is metabolized predominantly by carboxylesterases (not CYP3A4), and therefore has fewer CYP (cytochrome P450 enzyme)-based drug interactions than olaparib or rucaparib. Niraparib is distinguished by a higher rate of hematologic toxicity, particularly thrombocytopenia (occurring in approximately 61% of patients, grade 3 or 4 in approximately 29%), requiring weekly CBC (complete blood count) monitoring for the first month of therapy and dose reduction for significant thrombocytopenia. The class-wide toxicities of PARP inhibitors include anemia, nausea (most common; often manageable with antiemetics), fatigue, and a small but definite risk of treatment-emergent MDS (myelodysplastic syndrome) or AML, estimated at 1-2% with prolonged use. BRCA1/2 mutation testing (germline and somatic) is essential for proper patient selection; HRD testing (Myriad myChoice HRD test) provides additional selection information for ovarian cancer indication. Strong CYP3A4 inhibitors increase olaparib and rucaparib exposure and should prompt dose reduction.
FLT3 Inhibitors: Midostaurin and Gilteritinib. FLT3 (FMS-like tyrosine kinase 3, also designated CD135) is a transmembrane receptor tyrosine kinase expressed on early hematopoietic progenitors that normally promotes myeloid and dendritic cell differentiation in response to FLT3 ligand (FL).9 Activating FLT3 mutations occur in approximately 30% of AML (acute myeloid leukemia) cases, most commonly internal tandem duplications (FLT3-ITD) in the juxtamembrane domain (approximately 25%) and point mutations in the kinase domain (FLT3-TKD; approximately 7%). FLT3-ITD mutations confer a poor prognosis in AML and are a defined pharmacogenomic target for TKI (tyrosine kinase inhibitor) therapy. Midostaurin is a multi-kinase inhibitor (FLT3, KIT, PDGFR, PKC) approved for newly diagnosed FLT3-mutant AML in combination with standard daunorubicin-cytarabine induction chemotherapy (RATIFY trial), providing a significant overall survival benefit. Midostaurin is metabolized by CYP3A4 to active metabolites; strong CYP3A4 inhibitors (including posaconazole, commonly used for AML antifungal prophylaxis) increase midostaurin exposure and require careful monitoring. Gilteritinib is a selective FLT3 (FMS-like tyrosine kinase 3)/AXL (AXL receptor tyrosine kinase) inhibitor approved for relapsed or refractory FLT3-mutant AML; it is administered orally, metabolized by CYP3A4, and demonstrates activity against both FLT3-ITD and FLT3-TKD mutations. Both agents require QTc monitoring.
IDH1 (isocitrate dehydrogenase 1)/IDH2 (isocitrate dehydrogenase 2) Inhibitors and Differentiation Syndrome. IDH1 and IDH2 mutations occur in approximately 6-10% and 8-15% of AML cases, respectively, and produce a gain-of-function that generates the oncometabolite 2-hydroxyglutarate (2-HG), which inhibits TET2 (ten-eleven translocation methylcytosine dioxygenase 2) and other alpha-ketoglutarate-dependent dioxygenases, preventing normal myeloid differentiation and promoting a leukemic stem cell state.9 Ivosidenib (IDH1 inhibitor) and enasidenib (IDH2 inhibitor) are oral, selective inhibitors that suppress 2-HG production and restore differentiation capacity. The most important clinical complication unique to IDH inhibitors is differentiation syndrome (DS), a potentially life-threatening inflammatory syndrome analogous to the all-trans-retinoic acid (ATRA) syndrome seen in APL (acute promyelocytic leukemia). DS occurs in approximately 10-20% of patients and is thought to result from rapid maturation of leukemic blasts, causing cytokine release and an inflammatory state characterized by dyspnea, fever, hypotension, pulmonary infiltrates, pleural and pericardial effusions, AKI, and peripheral edema; it typically occurs within the first 5-12 weeks of therapy. Management requires prompt recognition and early initiation of systemic corticosteroids (dexamethasone 10 mg IV twice daily for at least 3 days or until resolution of symptoms); IDH inhibitor therapy is continued through mild-moderate differentiation syndrome but held for severe cases. Both IDH inhibitors also prolong the QTc interval, requiring baseline ECG (electrocardiogram) and periodic monitoring throughout therapy.
Differentiation syndrome during IDH inhibitor therapy is frequently misdiagnosed as infection, disease progression, or fluid overload. The clinical triad of dyspnea, unexplained fever, and bilateral pulmonary infiltrates in a patient receiving ivosidenib or enasidenib within the first 12 weeks of therapy should trigger immediate evaluation for DS. Dexamethasone 10 mg IV twice daily should be started without delay pending confirmation; late initiation of corticosteroids is associated with higher mortality. Diuresis may be needed for fluid overload component. IDH inhibitor should be held only for grade 4 or higher DS manifestations (severe renal failure, respiratory failure requiring ICU-level support); it may be continued for grade 2-3 DS with steroid coverage. Document and communicate DS events clearly as they affect future IDH inhibitor rechallenge decisions.
The ubiquitin-proteasome system (UPS) is the primary intracellular protein degradation machinery, responsible for eliminating misfolded, damaged, and regulatory proteins including IkB (the inhibitor of NF-kB), pro-apoptotic proteins, and cyclin-dependent kinase inhibitors. Multiple myeloma (MM) cells are uniquely dependent on proteasome function due to their high rate of immunoglobulin production, which generates a chronic burden of misfolded protein; proteasome inhibition creates an overwhelming unfolded protein response (UPR) leading to apoptosis specifically in these high-secretory plasma cells.
Bortezomib: First-Generation Proteasome Inhibitor. Bortezomib is a dipeptide boronic acid that reversibly inhibits the chymotrypsin-like (CT-L) activity of the 26S proteasome beta5 subunit by forming a slowly reversible covalent bond with the catalytic threonine residue.10 Its approved indications include multiple myeloma (newly diagnosed and relapsed/refractory) and mantle cell lymphoma. Bortezomib is administered intravenously or subcutaneously (SC); the subcutaneous route, delivering the drug into the adipose layer rather than directly into a vein, produces equivalent systemic exposure with a dramatically lower incidence of peripheral neuropathy (PN [peripheral neuropathy]; approximately 38% SC vs. 53% IV for any-grade PN; grade 3 or higher 6% SC vs. 16% IV), making SC administration the preferred route in all eligible patients. Bortezomib is metabolized by CYP3A4 (cytochrome P450 3A4) and CYP2C19 (cytochrome P450 2C19); strong CYP3A4 inhibitors increase exposure and CYP3A4 inducers reduce it. The key drug interaction is with strong CYP3A4 inhibitors: ketoconazole increases bortezomib AUC (area under the concentration-time curve) by approximately 35%; azole antifungal prophylaxis with agents such as posaconazole should be accompanied by bortezomib dose monitoring or preferably use of an alternative antifungal. Concomitant use of bortezomib with green tea extracts (EGCG [epigallocatechin gallate]; the principal polyphenol in green tea) is contraindicated in vitro and should be avoided clinically as EGCG can directly antagonize the boronic acid pharmacophore.
Bortezomib Peripheral Neuropathy. PN is the dose-limiting and cumulative toxicity of bortezomib, manifesting predominantly as a painful sensory neuropathy with or without autonomic features (orthostatic hypotension, constipation).10 The mechanism involves proteasome inhibition in dorsal root ganglion neurons and Schwann cells, disrupting axonal protein homeostasis, mitochondrial function, and NF-kB-dependent neuronal survival signaling. PN is graded using NCI (National Cancer Institute) CTCAE (Common Terminology Criteria for Adverse Events) for Adverse Events) criteria: grade 1 (asymptomatic), grade 2 (moderate symptoms, limiting instrumental ADL), grade 3 (severe symptoms, limiting self-care ADL), grade 4 (life-threatening, urgent intervention required). Dose modification guidelines are: grade 1 with pain or grade 2, reduce dose by 25%; grade 2 with pain or grade 3, hold until toxicity resolves to grade 1 or better, then reinitiate at 25% dose reduction; grade 4, permanently discontinue. PN is partially reversible upon dose reduction or discontinuation, but recovery can be slow (months) and incomplete. Duloxetine (a serotonin-norepinephrine reuptake inhibitor) has Level I evidence for management of painful chemotherapy-induced peripheral neuropathy (CIPN). Weekly (rather than twice-weekly) dosing of bortezomib in the maintenance or less-intensive setting substantially reduces PN incidence with comparable efficacy.
Carfilzomib: Second-Generation Proteasome Inhibitor. Carfilzomib is an irreversible, selective inhibitor of the CT-L activity of the 26S proteasome beta5 subunit; it forms an irreversible covalent bond with the catalytic threonine, providing sustained proteasome inhibition between doses that distinguishes it mechanistically from bortezomib’s reversible inhibition.11 Carfilzomib is administered intravenously (no oral or SC formulation) as a 10-30 minute infusion, initially twice weekly in 28-day cycles. Its major distinguishing toxicity is cardiovascular: carfilzomib causes cardiomyopathy and cardiac failure in approximately 7-25% of patients, hypertension in approximately 14%, and a higher rate of arterial thromboembolic events compared to bortezomib. The cardiovascular toxicity mechanism is not fully understood but may involve off-target inhibition of proteasome function in cardiomyocytes, endothelial cells, and platelets. Mandatory cardiac monitoring includes baseline cardiac evaluation (history, physical, ECG [electrocardiogram], and echocardiography if indicated), adequate hydration before infusions (250-500 mL IV normal saline before and after each dose), and dose interruption or discontinuation for grade 3 or higher cardiac events. Patients with NYHA (New York Heart Association) class III or IV heart failure are excluded from carfilzomib therapy. PN is substantially less common with carfilzomib than bortezomib due to its mechanism of irreversible inhibition, which paradoxically appears more favorable for neuronal preservation compared to bortezomib’s reversible inhibition pattern.
Ixazomib: Oral Proteasome Inhibitor. Ixazomib is the first oral proteasome inhibitor, a boronic acid analog that reversibly inhibits the beta5 subunit of the 26S proteasome, structurally related to bortezomib but with an oral formulation enabling outpatient, once-weekly dosing on days 1, 8, and 15 of a 28-day cycle.11 Oral bioavailability is approximately 58%; ixazomib is metabolized by multiple CYP (cytochrome P450) enzymes (CYP3A4, CYP1A2 [cytochrome P450 1A2], CYP2B6 [cytochrome P450 2B6], CYP2C8 [cytochrome P450 2C8], CYP2C19 [cytochrome P450 2C19], CYP2D6 [cytochrome P450 2D6]) and non-CYP hydrolases, making drug interactions with individual CYP inhibitors less dramatic than with bortezomib. The major advantage of ixazomib is its oral route, enabling combinations with oral immunomodulatory drugs (lenalidomide) and dexamethasone in outpatient settings; the IRd (ixazomib-lenalidomide-dexamethasone) triplet is approved for relapsed/refractory multiple myeloma. Tolerability is favorable compared to bortezomib and carfilzomib: PN is less frequent and less severe, and cardiovascular toxicity is not a prominent concern. Gastrointestinal toxicity (nausea, vomiting, diarrhea) and dermatologic reactions (rash) are the dominant adverse effects. Strong CYP3A4 inducers substantially reduce ixazomib exposure and should be avoided.
Bortezomib: preferred for newly diagnosed MM with renal impairment (its clearance is not renally dependent), for patients with aggressive presentations where rapid response is needed, and when cost is a constraint; always use SC route. Carfilzomib: preferred for relapsed/refractory MM after bortezomib failure, where deeper responses are needed; avoid in patients with pre-existing heart failure, uncontrolled hypertension, or prior anthracycline cardiomyopathy. Ixazomib: preferred when oral therapy is a priority (patient convenience, outpatient management), and when bortezomib neuropathy limits therapy; combines well with lenalidomide and dexamethasone. All three require antiviral prophylaxis (acyclovir or valacyclovir) against herpes zoster reactivation throughout therapy.
The immunomodulatory drugs (IMiDs) thalidomide, lenalidomide, and pomalidomide share a common molecular target and mechanism of action that was only elucidated decades after thalidomide’s clinical introduction. Their pharmacology encompasses antineoplastic, immunomodulatory, anti-angiogenic, and anti-inflammatory properties, along with a teratogenicity profile so severe that all three require mandatory risk evaluation and mitigation strategy (REMS) programs in the United States.
Mechanism: CRBN (Cereblon) and the CRL4 (Cullin-RING ligase 4) Ubiquitin Ligase. The IMiDs bind directly to CRBN (cereblon), the substrate receptor of the CRL4CRBN (Cullin-RING ligase 4-cereblon ubiquitin ligase) complex, a member of the ubiquitin ligase superfamily.12 This binding redirects the ubiquitin ligase to ubiquitinate and proteasomally degrade neo-substrates (proteins not normally degraded by this complex) including IKZF1 (Ikaros) and IKZF3 (Aiolos), transcription factors essential for myeloma cell survival and for IL-2 (interleukin-2) production in T cells; the depletion of Ikaros and Aiolos causes plasma cell death and simultaneously enhances T-cell and NK (natural killer) cell function, accounting for the combined direct anti-myeloma and immunostimulatory effects. In MDS (myelodysplastic syndrome) with isolated del(5q), the IMiD substrate CSNK1A1 (casein kinase 1 alpha 1) is degraded; because CSNK1A1 is encoded on the deleted chromosome 5q, MDS del(5q) cells are haploinsufficient for CSNK1A1 and cannot tolerate further reduction in its activity, making them selectively sensitive to lenalidomide-mediated CSNK1A1 degradation. The teratogenicity of thalidomide and lenalidomide results from CRBN-mediated degradation of PLZF (promyelocytic leukemia zinc finger protein) and SALL4 (sal-like transcription factor 4), transcription factors essential for limb bud and cardiac development.
Thalidomide: ADME (Absorption, Distribution, Metabolism, Excretion), Clinical Uses, and Toxicity. Thalidomide has oral bioavailability of approximately 90%, is non-renally cleared, and undergoes spontaneous hydrolysis and hepatic hydroxylation; it is not significantly metabolized by CYP (cytochrome P450) enzymes, making it less prone to CYP-based drug interactions than lenalidomide or pomalidomide.12 It is approved in combination with dexamethasone for newly diagnosed multiple myeloma in patients ineligible for autologous stem cell transplant. Its primary toxicities are peripheral neuropathy (cumulative, dose-limiting; predominantly sensory; monitor with clinical assessment at each visit), somnolence and constipation (common; manage with dose reduction and stimulant laxatives), and teratogenicity. The teratogenic risk of thalidomide is among the most severe in pharmacology: a single dose during the critical window of limb bud development (days 24-36 post-fertilization) can produce phocomelia (seal-limb deformity), amelia, and cardiac malformations; the absolute risk of fetal abnormality is approximately 20-50% in exposed pregnancies. The STEPS (System for Thalidomide Education and Prescribing Safety) REMS program requires mandatory registration of all prescribers and patients, two forms of contraception for women of childbearing potential, regular pregnancy testing, and a maximum 28-day prescription dispensing window.
Lenalidomide: ADME, Indications, and Toxicity. Lenalidomide is structurally related to thalidomide with substantially greater potency (approximately 50-100-fold), reduced neurologic toxicity, and a markedly different adverse effect profile.13 Oral bioavailability is approximately 65-70%, and the drug is predominantly renally eliminated unchanged (approximately 68% in urine); dose reduction is mandatory for CrCl (creatinine clearance) below 60 mL/min, and lenalidomide should not be used in patients with CrCl below 30 mL/min without hemodialysis.
Lenalidomide is approved for multiple myeloma (frontline as part of RVd [lenalidomide-bortezomib-dexamethasone] and maintenance post-autologous SCT (stem cell transplant)), MDS with isolated del(5q) (lenalidomide monotherapy; transfusion independence in approximately 70% of patients), and CLL (in combination regimens). The dominant toxicity is myelosuppression: neutropenia (grade 3 or 4 in approximately 30-50%) and thrombocytopenia (approximately 10-15%); CBC (complete blood count) monitoring is required weekly for the first 8 weeks, then monthly. Venous thromboembolism (VTE) is a class-wide risk of IMiDs, occurring in approximately 15-26% of patients receiving lenalidomide with dexamethasone without thromboprophylaxis; this risk is potentiated by dexamethasone, erythropoietin, and prior VTE history. VTE prophylaxis is mandatory: aspirin is appropriate for low-risk patients (IMiD monotherapy or with single-agent dexamethasone), and therapeutic anticoagulation (LMWH or DOAC) is required for high-risk patients (two or more risk factors, or combination with dexamethasone plus one additional agent). The REMS program (REVLIMID REMS) mirrors the thalidomide STEPS program in its contraception and pregnancy testing requirements.
Pomalidomide: Third-Generation IMiD. Pomalidomide is the most potent of the three IMiDs and retains activity in lenalidomide-refractory myeloma, attributed to more potent degradation of Ikaros and Aiolos at lower concentrations and to enhanced immunomodulatory effects.13 Oral bioavailability is approximately 73%, and metabolism is hepatic via CYP1A2 (cytochrome P450 1A2) and CYP3A4 (cytochrome P450 3A4), with P-gp playing a minor role; smoking induces CYP1A2 and may reduce pomalidomide exposure, requiring clinical awareness. Pomalidomide is approved in combination with dexamethasone for relapsed/refractory multiple myeloma after at least two prior therapies (including lenalidomide and a proteasome inhibitor). The toxicity profile is similar to lenalidomide, dominated by myelosuppression (particularly neutropenia) and VTE; mandatory thromboprophylaxis and REMS (POMALYST REMS) requirements parallel those of lenalidomide.
IMiD VTE Risk and Prophylaxis Algorithm. The mechanism of IMiD-associated VTE is multifactorial: the drugs increase procoagulant factor expression, reduce fibrinolysis, and promote endothelial adhesion of myeloma cells; dexamethasone independently increases VTE risk through endothelial activation and platelet aggregation.12 Risk stratification guides prophylaxis selection: patients receiving an IMiD as monotherapy or with low-dose dexamethasone only and no additional VTE risk factors may use aspirin 81-325 mg daily; all other patients (those receiving high-dose dexamethasone, multi-agent combinations, or with prior VTE, thrombophilia, immobility, or central venous catheter) require LMWH (low-molecular-weight heparin) or a DOAC (direct oral anticoagulant). Prophylaxis should be maintained throughout IMiD therapy. The interaction between lenalidomide and DOACs deserves attention: lenalidomide can increase DOAC exposure through P-gp inhibition, requiring appropriate DOAC dose selection.
All three IMiDs require a REMS program due to teratogenicity: thalidomide (STEPS), lenalidomide (REVLIMID REMS), and pomalidomide (POMALYST REMS). All programs require: prescriber and pharmacy registration; patient enrollment; two forms of contraception for women of childbearing potential with at least one highly effective method; pregnancy testing within 10-14 days before each prescription (for women of childbearing potential); and a 28-day dispensing limit per prescription. Male patients must also use condoms during therapy and for at least 1 week after the last dose. Prescribers must complete the REMS assessment before each prescription. Failure to comply with REMS requirements is a prescribing violation. Drug cannot be dispensed to patients not enrolled in the REMS program.
The breadth of agents in this module demands a systematic approach to toxicity recognition, drug interaction management, and companion diagnostic interpretation. The following section maps the highest-yield clinical decision points across all drug classes covered, organized to prepare for T3/T4 (higher-order clinical vignette)-level clinical reasoning.
Companion Diagnostics and Molecular Prerequisite Testing. Several drugs in this module require mandatory companion diagnostic testing before prescribing: alpelisib requires PIK3CA (phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha) mutation testing (tissue by therascreen or plasma by Guardant360 assay); PARP (poly-ADP-ribose polymerase) inhibitors in ovarian cancer require BRCA1/2 (breast cancer susceptibility gene 1/2) testing and, for expanded indication, HRD (homologous recombination deficiency) testing; FLT3 (FMS [FMS proto-oncogene receptor tyrosine kinase]-like tyrosine kinase 3) inhibitors require FLT3 mutation testing (ITD [internal tandem duplication] and TKD [tyrosine kinase domain]) in AML (acute myeloid leukemia); IDH (isocitrate dehydrogenase) inhibitors require IDH1 (ivosidenib) or IDH2 (enasidenib) mutation testing; venetoclax for AML is used in combination regimens but does not require a specific companion diagnostic; BRAF (v-raf murine sarcoma viral oncogene homolog B)/MEK (mitogen-activated extracellular signal-regulated kinase) inhibitors require BRAF V600E (valine-to-glutamate at codon 600) testing, preferably by a validated assay (cobas BRAF mutation test or next-generation sequencing).1489 Squamous cell carcinoma histology is a contraindication for BRAF inhibitor therapy in non-small cell lung cancer due to risk of accelerated growth from paradoxical MAPK (mitogen-activated protein kinase) activation in RAS (rat sarcoma viral proto-oncogene)-mutant squamous cells. For CRC (colorectal cancer) with BRAF V600E, RAS mutation testing is critical to confirm wild-type RAS before encorafenib-cetuximab therapy.
Hyperglycemia Management Across Drug Classes. Three distinct mechanisms of drug-induced hyperglycemia emerge from this module: PI3K (phosphoinositide 3-kinase)-alpha inhibition (alpelisib) causing insulin resistance; mTOR inhibition (everolimus, temsirolimus) causing impaired beta-cell compensation and insulin resistance; and corticosteroid-associated hyperglycemia (dexamethasone with IMiDs and bortezomib-based regimens).45 Alpelisib-associated hyperglycemia should be managed with dietary counseling, metformin (provided renal function allows), and insulin as escalating steps; starting patients on pre-emptive metformin before alpelisib initiation is a strategy used in clinical practice for patients with pre-diabetes or borderline glucose. Everolimus-associated hyperglycemia is typically less severe; monitoring fasting glucose before each cycle is standard. Steroid-induced hyperglycemia with dexamethasone tends to follow meal-related patterns and may require short-acting insulin at mealtimes rather than basal insulin adjustment. Patients with pre-existing diabetes require individualized management intensification before any of these drugs are initiated.
Perioperative Drug Management: A Multi-Drug Perspective. Multiple drugs in this module require perioperative planning. BTK (Bruton's tyrosine kinase) inhibitors (particularly ibrutinib) cause platelet dysfunction independent of thrombocytopenia and should be held for at least 3-7 days before major surgery; acalabrutinib and zanubrutinib carry lower platelet dysfunction risk but should also be held for 3 days pre-operatively.6 Venetoclax should be held perioperatively when GI (gastrointestinal) absorption may be compromised or when TLS (tumor lysis syndrome) risk is exacerbated by dehydration. IMiDs should be held before surgery because of VTE (venous thromboembolism) risk from immobility combined with drug effect; resumption after surgery requires re-assessment of VTE prophylaxis. Bortezomib and carfilzomib do not specifically require dose-holding for surgery but require attention to infection risk and adequate hydration (carfilzomib). PARP inhibitors should be held for at least 3 days pre-operatively and restarted after adequate wound healing given their potential for impaired DNA (deoxyribonucleic acid) repair in surgical wounds.
QTc Monitoring Across the Module. Multiple agents in this module prolong the QTc interval, creating cumulative risk in patients receiving combination regimens or polypharmacy for comorbidities. QTc-prolonging agents in this module include ribociclib (mandatory baseline and follow-up ECG protocol), enasidenib and ivosidenib (IDH inhibitors; ECG monitoring at baseline and monthly for the first 3 months), bortezomib (less frequent), and gilteritinib. Concurrent use of any of these agents with other QTc-prolonging drugs (antiarrhythmics, antipsychotics, azithromycin, fluoroquinolones, ondansetron at higher doses) requires careful ECG monitoring and, when possible, substitution of non-QTc-prolonging alternatives.39 For ribociclib specifically, the CDK4/6 (cyclin-dependent kinase 4/6) inhibitor with the most stringent QTc monitoring requirement, QTc above 481 ms requires dose interruption and QTc above 500 ms requires permanent discontinuation.
Herpes Zoster Prophylaxis and Infectious Complications. Proteasome inhibitors cause a marked increase in herpes zoster reactivation risk (VZV [varicella-zoster virus] reactivation), estimated at approximately 13% without prophylaxis; acyclovir (400 mg twice daily) or valacyclovir (500 mg once daily) is mandatory throughout bortezomib, carfilzomib, or ixazomib therapy and for at least 3 months after completion. IMiDs also increase VZV reactivation risk, particularly in combination with steroids or proteasome inhibitors, and antiviral prophylaxis should be co-prescribed. Idelalisib requires mandatory PJP (Pneumocystis jirovecii pneumonia) prophylaxis with trimethoprim-sulfamethoxazole (or dapsone or atovaquone for sulfa-allergic patients) and CMV (cytomegalovirus) monitoring throughout therapy. BTK inhibitors increase the risk of invasive fungal infections and atypical organisms including Pneumocystis; fungal prophylaxis should be considered in heavily pretreated patients or those with additional immunosuppression.4
BRAF V600E melanoma patient on vemurafenib developing new skin lesion: evaluate for cSCC or keratoacanthoma (paradoxical MAPK activation); biopsy if suspicious; add trametinib (BRAF/MEK combination is now standard). ER+/HER2- breast cancer patient on ribociclib-letrozole with QTc 495 ms: hold ribociclib immediately; QTc above 481 ms requires interruption; avoid all concurrent QTc-prolonging medications; do not restart if QTc exceeds 500 ms recurrently. CLL patient on ibrutinib develops AF requiring anticoagulation: avoid warfarin due to ibrutinib-CYP2C9 interaction; use DOAC with awareness of P-gp inhibition; consider switching to acalabrutinib or zanubrutinib. AML patient on enasidenib presenting at week 6 with dyspnea, fever, and bilateral infiltrates: hold enasidenib if severe; start dexamethasone 10 mg IV twice daily immediately for suspected differentiation syndrome; do not mistake for pneumonia without corticosteroid cover. Newly diagnosed myeloma patient on lenalidomide-bortezomib-dexamethasone with leg swelling and positive D-dimer: VTE is an expected complication; initiate LMWH or DOAC; do not discontinue lenalidomide without oncology consultation. Venetoclax initiation in CLL: strict ramp-up schedule required; withhold strong CYP3A4 inhibitors during ramp-up; allopurinol prophylaxis, aggressive hydration, and 6-8 hour post-dose laboratory monitoring are mandatory.
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