Renal Pharmacology

Correct Answer

C — Tacrolimus

Rationale

Tacrolimus is a calcineurin inhibitor that forms a complex with the immunophilin FKBP12 to inhibit calcineurin, blocking interleukin-2 gene transcription and T-cell activation. Sirolimus is an mTOR inhibitor. Mycophenolate mofetil is an antiproliferative agent that inhibits inosine monophosphate dehydrogenase. Prednisone is a corticosteroid.

Correct Answer

A — Mycophenolate mofetil

Rationale

Mycophenolate mofetil is a prodrug hydrolyzed to mycophenolic acid, which uncompetitively inhibits inosine monophosphate dehydrogenase — the rate-limiting enzyme in de novo guanosine synthesis. Because T and B lymphocytes rely almost exclusively on this pathway, the drug selectively suppresses lymphocyte proliferation. Tacrolimus and cyclosporine are calcineurin inhibitors. Sirolimus is an mTOR inhibitor. Basiliximab is an anti-CD25 monoclonal antibody.

Correct Answer

D — Azathioprine

Rationale

Azathioprine is a thiopurine prodrug converted to 6-mercaptopurine and then to active thioguanine nucleotides that incorporate into replicating deoxyribonucleic acid, causing cytotoxicity in rapidly dividing cells including lymphocytes. Mycophenolate mofetil is also a prodrug but belongs to the antiproliferative class as an inosine monophosphate dehydrogenase inhibitor, not a thiopurine. Sirolimus is an mTOR inhibitor. Basiliximab is a monoclonal antibody.

Correct Answer

B — Sirolimus

Rationale

Sirolimus is an mTOR inhibitor that, like tacrolimus, binds FKBP12 — but the sirolimus-FKBP12 complex inhibits mechanistic target of rapamycin complex 1 rather than calcineurin, blocking cytokine-driven T-cell proliferation at the G1-to-S phase transition. Tacrolimus is a calcineurin inhibitor. Mycophenolate mofetil is an inosine monophosphate dehydrogenase inhibitor. Azathioprine is a thiopurine antiproliferative agent.

Correct Answer

C — Basiliximab

Rationale

Basiliximab is a chimeric monoclonal antibody directed against CD25, the interleukin-2 receptor alpha chain expressed on activated T cells. By blocking CD25, it interrupts the autocrine interleukin-2 loop driving T-cell clonal expansion. Antithymocyte globulin is a polyclonal antibody preparation that depletes T cells by complement-mediated lysis and apoptosis. Tacrolimus is a calcineurin inhibitor. Sirolimus is an mTOR inhibitor.

Correct Answer

A — Methylprednisolone

Rationale

Intravenous pulse methylprednisolone is the first-line treatment for acute T-cell mediated rejection, given at 500 milligrams intravenously daily for three consecutive days. As a corticosteroid, it suppresses T-cell activation by inhibiting nuclear factor kappa B and blocking transcription of interleukin-1, interleukin-6, and tumor necrosis factor-alpha. Tacrolimus and sirolimus are maintenance immunosuppressants. Mycophenolate mofetil is an antiproliferative maintenance agent.

Correct Answer

B — Tacrolimus binds FKBP12, and the complex inhibits calcineurin, preventing dephosphorylation and nuclear translocation of nuclear factor of activated T cells and blocking interleukin-2 transcription

Rationale

Tacrolimus binds the immunophilin FKBP12; the tacrolimus-FKBP12 complex inhibits calcineurin, a phosphatase required to dephosphorylate nuclear factor of activated T cells. Without this dephosphorylation, nuclear factor of activated T cells cannot translocate to the nucleus, and interleukin-2 gene transcription is blocked — halting T-cell activation. Cyclosporine uses the same downstream calcineurin target but binds cyclophilin rather than FKBP12. Sirolimus binds FKBP12 but targets mTOR complex 1, not calcineurin. Basiliximab blocks the interleukin-2 receptor alpha chain.

Correct Answer

D — Fluconazole inhibits cytochrome P450 3A4, reducing tacrolimus metabolism and raising its blood levels

Rationale

Tacrolimus is a substrate of cytochrome P450 3A4 and P-glycoprotein. Fluconazole and other azole antifungals are potent inhibitors of cytochrome P450 3A4, reducing the hepatic and intestinal metabolism of tacrolimus and raising its blood levels two- to fivefold. Because tacrolimus has a narrow therapeutic index, this interaction can rapidly produce nephrotoxicity or neurotoxicity. Therapeutic drug monitoring with trough levels must be performed within days of starting or stopping any azole antifungal in a transplant patient on calcineurin inhibitors.

Correct Answer

A — Calcineurin inhibitors cause afferent arteriolar vasoconstriction via thromboxane A2 and endothelin, producing sustained ischemia that leads to interstitial fibrosis and tubular atrophy

Rationale

Calcineurin inhibitor nephrotoxicity operates through afferent arteriolar vasoconstriction mediated by thromboxane A2 and endothelin, reducing renal blood flow and glomerular filtration rate. Acute nephrotoxicity from this mechanism is dose-dependent and reversible. Chronic nephrotoxicity results from sustained ischemia producing irreversible interstitial fibrosis and tubular atrophy — the histological pattern described in this patient. This progressive structural injury is a primary driver of calcineurin inhibitor minimization strategies in transplant medicine.

Correct Answer

C — Cyclosporine causes gingival hyperplasia, hirsutism, and hyperlipidemia; tacrolimus causes post-transplant diabetes mellitus and neurotoxicity including tremor

Rationale

Despite sharing the same downstream target — calcineurin inhibition — tacrolimus and cyclosporine have divergent adverse effect profiles because they bind different intracellular immunophilins. Cyclosporine (cyclophilin binding) causes gingival hyperplasia, hirsutism, and hyperlipidemia. Tacrolimus (FKBP12 binding) carries a higher risk of post-transplant diabetes mellitus through pancreatic beta-cell toxicity and peripheral insulin resistance, and a higher risk of neurotoxicity including tremor and posterior reversible encephalopathy syndrome. Both agents cause nephrotoxicity and hypertension. For a male patient concerned about cosmetic effects, tacrolimus avoids the gingival hyperplasia and hirsutism associated with cyclosporine.

Correct Answer

B — Lymphocytes rely almost exclusively on the de novo purine synthesis pathway, lacking the salvage pathway capacity that allows most other cells to maintain guanosine nucleotide pools when de novo synthesis is blocked

Rationale

Most rapidly dividing cells can maintain purine nucleotide pools through two pathways: de novo synthesis and a salvage pathway that recycles purine bases from nucleotide degradation. Lymphocytes lack sufficient salvage pathway capacity and depend almost entirely on de novo guanosine synthesis. When mycophenolic acid blocks inosine monophosphate dehydrogenase — the rate-limiting step in de novo guanosine synthesis — lymphocytes are selectively deprived of the guanosine nucleotides required for deoxyribonucleic acid replication and cannot proliferate, while most other cell types use the salvage pathway to compensate.

Correct Answer

D — Allopurinol inhibits xanthine oxidase, which normally metabolizes 6-mercaptopurine; with xanthine oxidase blocked, 6-mercaptopurine and its toxic thioguanine nucleotide metabolites accumulate to myelosuppressive levels

Rationale

Azathioprine is converted to 6-mercaptopurine, which is normally inactivated in part by xanthine oxidase. Allopurinol inhibits xanthine oxidase to treat gout, but this simultaneously blocks 6-mercaptopurine catabolism, causing it and its active thioguanine nucleotide metabolites to accumulate to toxic concentrations. The result is severe bone marrow suppression with pancytopenia. This interaction requires either an azathioprine dose reduction of 75% or substitution with mycophenolate mofetil when a xanthine oxidase inhibitor is clinically required.

Correct Answer

A — mTOR inhibitors impair wound healing as a class effect, and initiating them before surgical wounds are healed increases the risk of wound dehiscence and serious complications

Rationale

Wound healing impairment is a class effect of mTOR inhibitors, arising because mTOR signaling is required for the cellular proliferation and protein synthesis involved in tissue repair. Initiating sirolimus or everolimus before surgical wounds have healed — typically within the first three months after transplantation — substantially increases the risk of wound dehiscence, lymphocele, and impaired incision healing. mTOR inhibitors are therefore started no earlier than three months post-transplant when healing is complete, and are typically introduced as part of a calcineurin inhibitor minimization strategy at that stage.

Correct Answer

C — Basiliximab competitively blocks the interleukin-2 receptor alpha chain on activated T cells, interrupting the autocrine interleukin-2 loop that drives clonal expansion

Rationale

Basiliximab is a chimeric monoclonal antibody directed against CD25, the interleukin-2 receptor alpha chain expressed on activated T cells. By blocking CD25, basiliximab prevents interleukin-2 from binding its high-affinity receptor, interrupting the autocrine signaling loop through which activated T cells stimulate their own proliferation. This provides T-cell suppression for four to six weeks with an excellent tolerability profile and minimal infusion reactions, making it the preferred induction agent for standard-risk transplant recipients. T-cell depletion by complement-mediated lysis is the mechanism of antithymocyte globulin, not basiliximab.

Correct Answer

D — Voriconazole inhibits cytochrome P450 3A4, blocking tacrolimus metabolism and raising its blood levels to toxic concentrations

Rationale

Tacrolimus is primarily metabolized by cytochrome P450 3A4 in the liver and intestinal wall. Voriconazole, like other azole antifungals, is a potent inhibitor of cytochrome P450 3A4 and can increase tacrolimus blood levels two- to fivefold within days of initiation. Because tacrolimus has a narrow therapeutic index, this interaction rapidly produces neurotoxicity (tremor, encephalopathy) and nephrotoxicity. Tacrolimus trough levels must be checked within 24 to 48 hours of starting or stopping any azole antifungal, and the tacrolimus dose must be preemptively reduced — often by 50 to 75% — when voriconazole is initiated.

Correct Answer

B — Pulse intravenous methylprednisolone, which suppresses T-cell activation by inhibiting nuclear factor kappa B and blocking pro-inflammatory cytokine transcription

Rationale

This patient has biopsy-confirmed T-cell mediated rejection, characterized by lymphocytic tubulitis and interstitial inflammation. First-line treatment is pulse intravenous methylprednisolone at 500 milligrams daily for three consecutive days. Corticosteroids suppress the T-cell-driven alloimmune response by inhibiting nuclear factor kappa B, blocking transcription of interleukin-1, interleukin-6, and tumor necrosis factor-alpha required for T-cell activation and effector function. Most T-cell mediated rejection episodes respond to pulse steroids. Rituximab, plasmapheresis, and intravenous immunoglobulin are used for antibody-mediated rejection, which has a different histological signature including peritubular capillary inflammation and complement component 4d deposition.

Correct Answer

A — Allopurinol inhibits xanthine oxidase, blocking catabolism of 6-mercaptopurine derived from azathioprine, causing thioguanine nucleotide accumulation and severe myelosuppression

Rationale

Azathioprine is converted to 6-mercaptopurine, which is normally catabolized in part by xanthine oxidase. Allopurinol blocks xanthine oxidase to reduce uric acid production, but simultaneously prevents 6-mercaptopurine breakdown, causing it and its active thioguanine nucleotide metabolites to accumulate to myelosuppressive concentrations. The result is potentially life-threatening pancytopenia. When a xanthine oxidase inhibitor is required in a patient on azathioprine, the azathioprine dose must be reduced by approximately 75%, or the patient should be switched to mycophenolate mofetil, which does not share this interaction.

Correct Answer

C — Gingival hyperplasia and hirsutism are adverse effects specific to cyclosporine related to its binding of cyclophilin in non-immune tissues; tacrolimus does not produce these effects because it binds a different immunophilin

Rationale

Although tacrolimus and cyclosporine both inhibit calcineurin downstream, they bind different intracellular immunophilins — cyclosporine binds cyclophilin, tacrolimus binds FKBP12. The cosmetic adverse effects of gingival hyperplasia and hirsutism are specific to cyclosporine and are thought to result from cyclophilin binding and calcineurin inhibition in gingival fibroblasts and hair follicle cells. Tacrolimus does not produce these effects. Switching to tacrolimus resolves or prevents gingival hyperplasia and hirsutism while maintaining calcineurin-based immunosuppression, making it the appropriate choice for this patient.