Immunopharmacology: Foundations and Cytokine Pharmacology

Correct Answer

B — Tacrolimus

Rationale

Tacrolimus is classified as a calcineurin inhibitor. Along with cyclosporine, it belongs to this class of immunosuppressants that act by blocking calcineurin, a phosphatase required for interleukin-2 gene transcription in activated T cells. Sirolimus is classified as an mTOR inhibitor. Mycophenolate mofetil is classified as an antimetabolite and inosine monophosphate dehydrogenase inhibitor. Azathioprine is classified as a thiopurine antimetabolite.

Correct Answer

A — Mycophenolate mofetil

Rationale

Mycophenolate mofetil is a prodrug that is hydrolyzed to mycophenolic acid, which is classified as an inhibitor of inosine monophosphate dehydrogenase, the rate-limiting enzyme in the de novo purine synthesis pathway. Sirolimus is an mTOR inhibitor. Tacrolimus is a calcineurin inhibitor that acts through FK-binding protein 12. Basiliximab is an anti-CD25 monoclonal antibody that targets the interleukin-2 receptor alpha chain.

Correct Answer

D — Sirolimus

Rationale

Sirolimus is classified as a mammalian target of rapamycin inhibitor. Along with everolimus, it belongs to the mTOR inhibitor class of immunosuppressants. Tacrolimus and cyclosporine are calcineurin inhibitors; they bind FKBP-12 and cyclophilin respectively, but inhibit calcineurin, not mTOR. Azathioprine is a thiopurine antimetabolite and prodrug of 6-mercaptopurine.

Correct Answer

B — Basiliximab

Rationale

Basiliximab is a chimeric anti-CD25 monoclonal antibody. CD25 is the interleukin-2 receptor alpha chain, and basiliximab is used as an induction agent in standard-immunological-risk solid organ transplant recipients. Anti-thymocyte globulin is a polyclonal antibody preparation directed against multiple T-cell surface antigens and is used for high-risk induction or steroid-resistant rejection, not classified as anti-CD25. Rituximab targets CD20 on B cells. Mycophenolate mofetil is an antimetabolite, not an induction antibody.

Correct Answer

A — Azathioprine

Rationale

Azathioprine is a thiopurine prodrug that is cleaved to 6-mercaptopurine after administration. Mycophenolate mofetil is also a prodrug — it is hydrolyzed to mycophenolic acid — but it belongs to the antimetabolite class and is an IMPDH inhibitor, not a thiopurine. Sirolimus is an mTOR inhibitor and is not a prodrug in this sense. Tacrolimus is a calcineurin inhibitor that binds FKBP-12 directly without prodrug conversion.

Correct Answer

D — Cyclosporine

Rationale

Cyclosporine is the only calcineurin inhibitor with regulatory approval that extends beyond transplantation to include severe plaque psoriasis, rheumatoid arthritis refractory to methotrexate, and atopic dermatitis. This broad approval reflects its established track record since the 1970s and its well-characterized dosing in non-transplant immunological conditions. Tacrolimus is approved for solid organ transplantation and has a topical formulation for atopic dermatitis, but is not approved as a systemic agent for psoriasis or rheumatoid arthritis. Sirolimus is approved in transplantation and select rare diseases but not in psoriasis or rheumatoid arthritis. Mycophenolate mofetil is approved for transplantation and used off-label in some autoimmune conditions but lacks regulatory approval for psoriasis or rheumatoid arthritis.

Correct Answer

C — Tacrolimus is more diabetogenic than cyclosporine because it more potently impairs pancreatic beta-cell insulin secretion

Rationale

New-onset diabetes after transplantation occurs in 10 to 20% of patients receiving tacrolimus-based regimens, compared to 5 to 10% with cyclosporine. Both calcineurin inhibitors impair glucose-stimulated insulin secretion from pancreatic beta cells, but tacrolimus does so more potently, reducing insulin secretory capacity to a greater degree than cyclosporine at equivalent immunosuppressive doses. This comparative difference is a recognized clinical consideration when selecting a calcineurin inhibitor for patients at elevated risk of diabetes — such as those with pre-diabetes, obesity, or a family history of type 2 diabetes — where cyclosporine may be preferred. Both agents are diabetogenic independent of corticosteroid use, though steroids further increase the risk.

Correct Answer

A — Rifampin

Rationale

Rifampin is a potent inducer of drug-metabolizing enzymes and markedly increases the breakdown of tacrolimus and cyclosporine, causing trough concentrations to fall below the therapeutic range within days. This reduction in calcineurin inhibitor exposure can precipitate acute allograft rejection. When rifampin cannot be avoided in a transplant recipient, tacrolimus doses typically must be increased substantially and trough concentrations monitored frequently. Isoniazid, ethambutol, and pyrazinamide do not have significant inducing effects on tacrolimus metabolism and are preferred agents for latent tuberculosis treatment in transplant recipients for this reason.

Correct Answer

C — Allopurinol inhibits xanthine oxidase, blocking catabolism of 6-mercaptopurine and causing four-fold accumulation of toxic thioguanine nucleotides

Rationale

Azathioprine is a prodrug cleaved to 6-mercaptopurine. Xanthine oxidase normally catabolizes 6-mercaptopurine to inactive thiouric acid. When allopurinol or febuxostat blocks xanthine oxidase, this catabolic route is closed, shunting 6-mercaptopurine into the anabolic pathway via hypoxanthine-guanine phosphoribosyltransferase to generate thioguanine nucleotides. Thioguanine nucleotides accumulate approximately four-fold, producing severe myelosuppression with pancytopenia that can be fatal. This combination is absolutely contraindicated at standard azathioprine doses. If coadministration is unavoidable, the azathioprine dose must be reduced by 67 to 75% with weekly complete blood count monitoring.

Correct Answer

A — Lymphocytes are uniquely dependent on the de novo purine synthesis pathway and cannot effectively use the purine salvage pathway

Rationale

Most cell types can maintain adequate purine nucleotide pools by salvaging hypoxanthine and guanine from nucleotide breakdown products through hypoxanthine-guanine phosphoribosyltransferase. Lymphocytes are unusual in that they cannot effectively use this salvage pathway and are therefore uniquely dependent on the de novo synthesis of guanosine monophosphate. Because inosine monophosphate dehydrogenase is the rate-limiting enzyme in de novo guanosine nucleotide synthesis, mycophenolic acid — which inhibits this enzyme — selectively depletes guanosine nucleotide pools in lymphocytes. This selective dependence is the pharmacological basis for mycophenolate mofetil's immunosuppressive specificity.

Correct Answer

B — mTOR inhibition blocks fibroblast proliferation, impairing the cellular phase of wound healing

Rationale

Mammalian target of rapamycin complex 1 (mTORC1) drives cell cycle progression from G1 to S phase in fibroblasts and other proliferating cells. Sirolimus and everolimus inhibit mTORC1, which suppresses fibroblast proliferation and delays the cellular proliferative phase of wound healing. This increases the risk of wound dehiscence, lymphocele formation, and incisional complications. For this reason, mTOR inhibitors are generally withheld for 4 to 12 weeks after transplant surgery and are not used as primary immunosuppressants in the immediate post-operative period.

Correct Answer

D — Whole blood collected immediately before the morning dose to measure trough concentration

Rationale

Tacrolimus therapeutic drug monitoring requires whole blood — not plasma or serum — because tacrolimus distributes extensively into red blood cells. Using plasma or serum would substantially underestimate true drug exposure and lead to inappropriate dose adjustments. Samples are collected as trough concentrations, drawn immediately before the next scheduled dose, which is the standard monitoring parameter for calcineurin inhibitors. Cyclosporine is monitored the same way. Peak concentrations are not used for routine tacrolimus monitoring.

Correct Answer

D — Basiliximab saturates the interleukin-2 receptor alpha chain, preventing interleukin-2-driven T-cell proliferation for approximately 4 to 6 weeks

Rationale

Basiliximab is a chimeric anti-CD25 monoclonal antibody. CD25 is the interleukin-2 receptor alpha chain, which combines with the beta and gamma chains to form the high-affinity interleukin-2 receptor on activated T cells. By saturating CD25, basiliximab prevents interleukin-2 from engaging its high-affinity receptor complex, blocking the interleukin-2-driven T-cell proliferative signal that would otherwise drive early rejection. Two fixed doses given on day 0 and day 4 maintain receptor saturation for approximately 4 to 6 weeks, covering the highest-risk period for acute cellular rejection. Basiliximab does not deplete T cells and does not affect calcineurin or co-stimulatory pathways directly.

Correct Answer

A — Calcineurin inhibitors cause dose-dependent afferent arteriolar vasoconstriction, reducing glomerular filtration rate

Rationale

Calcineurin inhibitors cause two distinct forms of nephrotoxicity. The acute form — illustrated here — is functional and reversible: calcineurin inhibitors constrict the afferent arteriole, reducing glomerular filtration pressure and glomerular filtration rate in a dose-dependent manner. Creatinine rises within days to weeks and normalizes with dose reduction. This functional nephrotoxicity does not reflect permanent structural damage. The chronic form, by contrast, involves irreversible interstitial fibrosis and tubular atrophy (calcineurin inhibitor nephropathy) that does not resolve with dose reduction. Distinguishing these two mechanisms is critical because the treatment differs: dose reduction resolves acute functional toxicity, while chronic structural toxicity requires consideration of calcineurin inhibitor minimization or withdrawal.

Correct Answer

B — Tacrolimus inhibits calcineurin, blocking interleukin-2 transcription and preventing the T-cell activation and proliferation required for rejection

Rationale

Tacrolimus is a calcineurin inhibitor. Calcineurin is a phosphatase that, when activated following T-cell receptor engagement, drives expression of interleukin-2 and other T-cell activation genes. By inhibiting calcineurin, tacrolimus prevents interleukin-2 transcription, which in turn blocks T-cell activation and the clonal proliferation that would otherwise lead to allograft rejection. Tacrolimus does not deplete T cells and does not act on the interleukin-2 receptor — that is the mechanism of basiliximab. Inhibiting mTOR to block T-cell cycle progression is the mechanism of sirolimus and everolimus.

Correct Answer

B — Voriconazole inhibits CYP3A4, reducing tacrolimus metabolism and causing tacrolimus blood concentrations to rise to toxic levels

Rationale

Tacrolimus is extensively metabolized by cytochrome P450 3A4 in the liver and intestinal wall. Voriconazole, like all triazole antifungals, is a potent CYP3A4 inhibitor; coadministration can raise tacrolimus concentrations several-fold within days, causing nephrotoxicity (afferent arteriolar vasoconstriction) and neurotoxicity (tremor, and at higher levels, posterior reversible encephalopathy syndrome). When voriconazole is initiated in a transplant recipient on a calcineurin inhibitor, the tacrolimus dose must be reduced pre-emptively — typically by 50 to 75% — with intensive trough monitoring every 48 to 72 hours until stable concentrations are reestablished.

Correct Answer

D — Switch to azathioprine, which is the preferred antimetabolite when immunosuppression is required during pregnancy

Rationale

Mycophenolate mofetil is a potent human teratogen. It causes cleft palate, ear abnormalities, limb hypoplasia, and cardiac defects in up to 25% of exposed pregnancies. All women of childbearing potential must use reliable contraception during therapy, and mycophenolate mofetil must be discontinued before conception. Azathioprine is the preferred antimetabolite when maintenance immunosuppression is required during pregnancy; it has a long record of use in pregnant transplant recipients and does not carry the same structural teratogenicity as mycophenolate mofetil. Sirolimus is also avoided in pregnancy due to insufficient safety data and known anti-proliferative effects on fetal tissue.

Correct Answer

A — Supratherapeutic tacrolimus concentrations impair cerebrovascular endothelial function, causing loss of autoregulation and cerebral edema

Rationale

Posterior reversible encephalopathy syndrome (PRES) is a recognized complication of calcineurin inhibitor toxicity, occurring more frequently with tacrolimus than cyclosporine. At supratherapeutic concentrations, tacrolimus impairs calcineurin-dependent signaling in cerebrovascular endothelial cells, disrupting autoregulatory tone and causing vasospasm alternating with vasodilation, endothelial dysfunction, and subsequent breakdown of the blood-brain barrier. This produces the characteristic posterior predominant vasogenic edema seen on MRI. Clinical manifestations include confusion, visual disturbance, seizures, and headache. PRES is largely reversible with dose reduction or drug discontinuation and return of tacrolimus concentrations to the therapeutic range.