Immunopharmacology · Module 2 of 5

Transplant Immunosuppression

Calcineurin inhibitors, mTOR inhibitors, antimetabolites, and transplant protocols

Calcineurin inhibitors block T-cell activation at its earliest intracellular step: cyclosporine binds cyclophilin and tacrolimus binds FKBP-12, but both drug-immunophilin complexes inhibit calcineurin, the phosphatase that dephosphorylates NFAT, preventing IL-2 transcription and halting T-cell proliferation; tacrolimus is ~100-fold more potent and has replaced cyclosporine in most transplant protocols.
Both CNIs share nephrotoxicity (functional afferent arteriolar vasoconstriction and chronic structural fibrosis), hypertension, hypomagnesemia, and hyperkalemia; cyclosporine adds hirsutism, gingival hyperplasia, and hyperlipidemia; tacrolimus adds NODAT (10–20%), neurotoxicity (tremor in up to 40%), and PRES at toxic levels; both require whole-blood trough monitoring (C0 in EDTA tube, not plasma).
mTOR inhibitors (sirolimus, everolimus) bind FKBP-12 like tacrolimus but their FKBP-12 complex inhibits mTORC1, not calcineurin, blocking T-cell proliferation downstream of IL-2; they enable CNI dose reduction but add class-specific toxicities: pulmonary toxicity (sirolimus pneumonitis 3–11%), impaired wound healing (avoid first 4–12 weeks post-surgery), and hypertriglyceridemia.
Azathioprine’s key pharmacogenomic hazard: TPMT poor metabolizers (~0.3% homozygous) accumulate thioguanine nucleotides at toxic levels causing fatal pancytopenia; the most dangerous interaction is allopurinol or febuxostat (xanthine oxidase inhibitors), which block 6-MP catabolism and cause four-fold TGN accumulation — absolutely contraindicated unless azathioprine dose is reduced 67–75%; MMF is the preferred antimetabolite but is a potent teratogen requiring reliable contraception.
Standard triple therapy pairs tacrolimus + MMF + prednisone; induction at transplant uses basiliximab (anti-CD25, standard risk) or anti-thymocyte globulin (high risk); acute cellular rejection is treated with pulse methylprednisolone ± ATG; antibody-mediated rejection (worse prognosis, leading cause of late graft loss) requires plasmapheresis + IVIG + rituximab.

AMR = antibody-mediated rejection · ATG = anti-thymocyte globulin · CNI = calcineurin inhibitor · CMV = cytomegalovirus · CYP3A4 = cytochrome P450 3A4 · DSA = donor-specific antibody · FKBP-12 = FK-binding protein 12 · IMPDH = inosine monophosphate dehydrogenase · MMF = mycophenolate mofetil · mTOR = mammalian target of rapamycin · NFAT = nuclear factor of activated T cells · NODAT = new-onset diabetes after transplantation · PRES = posterior reversible encephalopathy syndrome · TGN = thioguanine nucleotide · TPMT = thiopurine methyltransferase

Calcineurin Inhibitor Mechanism

T-cell receptor

Antigen recognition

→

Calcium rise

Calmodulin activated

→

Calcineurin

Dephosphorylates NFAT

→

NFAT nucleus

IL-2 gene transcription

■

CNI block

Cyclosporine–cyclophilin or tacrolimus–FKBP-12 inhibits calcineurin

Cyclosporine vs. Tacrolimus

Feature	Cyclosporine	Tacrolimus
Immunophilin	Cyclophilin	FKBP-12
Potency	Standard reference	~100-fold more potent; now first-line for most organs
Distinct AEs	Hirsutism, gingival hyperplasia, hyperlipidemia (elevated LDL), hyperuricemia	NODAT 10–20%, neurotoxicity (tremor ~40%), PRES at toxic levels, alopecia
Shared AEs	Nephrotoxicity (functional + structural), hypertension, hypomagnesemia, hyperkalemia, thrombotic microangiopathy (rare)
Metabolism	CYP3A4 + CYP3A5 + P-glycoprotein; extensive erythrocyte distribution → whole-blood monitoring required
TDM target	C0 trough (EDTA whole blood)	C0 trough; early: 8–12 ng/mL; maintenance: 5–8 ng/mL (kidney)

mTOR Inhibitors

Sirolimus / Everolimus

mTORC1 Inhibition

Bind FKBP-12 like tacrolimus, but sirolimus–FKBP-12 complex inhibits mTORC1, not calcineurin
Block G1→S phase progression: suppress T-cell proliferative response to IL-2 (downstream of CNI target)
Enable CNI dose reduction to preserve renal function
Sirolimus t½ ~60 h (once daily); everolimus t½ ~28–30 h (twice daily); CYP3A4 + P-gp substrate (same interaction profile as CNIs)

mTOR Inhibitor Class Toxicities

Class-Specific Risks

Pulmonary toxicity: sirolimus pneumonitis 3–11%; ranges from asymptomatic infiltrates to organizing pneumonia → drug discontinuation
Impaired wound healing: inhibit fibroblast proliferation → dehiscence risk; avoid first 4–12 weeks post-transplant surgery
Hypertriglyceridemia (prominent), peripheral edema, oral mucositis
Therapeutic drug monitoring required (trough in whole blood)

Antimetabolites

Azathioprine

Thiopurine Prodrug

Prodrug → 6-mercaptopurine → TGN via HGPRT (active immunosuppressant)
Xanthine oxidase catabolizes 6-MP → inactive thiouric acid
Allopurinol / febuxostat absolutely contraindicated at standard dose: XO block → four-fold TGN accumulation → fatal pancytopenia
TPMT poor metabolizers (~0.3%): severe myelotoxicity at standard doses — genotype or phenotype before prescribing
NUDT15 polymorphisms: additional myelotoxicity risk especially in patients of East Asian ancestry
Safe in pregnancy (unlike MMF); preferred antimetabolite when immunosuppression required in pregnancy

Mycophenolate Mofetil (MMF)

IMPDH Inhibitor

Prodrug → mycophenolic acid (MPA); inhibits IMPDH, rate-limiting enzyme in de novo purine synthesis
Lymphocytes lack salvage pathway → selectively suppresses lymphocyte proliferation
Enterohepatic recirculation → secondary plasma peak at 6–12 hours
GI toxicity (nausea, diarrhea, cramping): 30–45%; often dose-dependent; switch to enteric-coated MPA sodium if needed
Teratogen: cleft palate, ear/limb/cardiac defects in up to 25% exposed pregnancies; reliable contraception mandatory
Preferred over azathioprine in most transplant protocols (superior efficacy, better hematological profile)

Standard Transplant Protocol

Induction

At Transplant

Standard risk: basiliximab (anti-CD25) 20 mg IV day 0 + day 4; saturates IL-2Rα for ~4–6 weeks; well tolerated
High risk: ATG (rabbit thymoglobulin) 1.5 mg/kg/day IV × 3–7 days; causes profound T-cell depletion
ATG pre-medication: corticosteroid + antihistamine + acetaminophen (cytokine release syndrome prevention)

Maintenance

Triple Therapy

Tacrolimus (CNI) — blocks IL-2 production at calcineurin
MMF — blocks proliferative response by depleting purine precursors
Prednisone — suppresses cytokine milieu and co-stimulatory signals
Steroid taper to 5–10 mg/day by 3–6 months post-transplant

Rejection Treatment

ACR vs. AMR

ACR (T-cell mediated, Banff I–III): pulse methylprednisolone 250–1,000 mg IV × 3–5 days
Steroid-resistant ACR: ATG
AMR (donor-specific antibody, complement + NK ADCC): plasmapheresis + high-dose IVIG (2 g/kg) + rituximab
AMR prognosis worse than ACR; leading cause of late kidney allograft failure

CYP3A4 Interaction Rule — All CNIs and mTOR Inhibitors

All CNIs (cyclosporine, tacrolimus) and mTOR inhibitors (sirolimus, everolimus) are CYP3A4 + P-glycoprotein substrates and share the same drug interaction profile. Strong inhibitors — azole antifungals (fluconazole, voriconazole, itraconazole, posaconazole), clarithromycin — raise drug levels several-fold within days and require pre-emptive dose reduction with intensive trough monitoring. Strong inducers — rifampin (most dangerous), carbamazepine, phenytoin, phenobarbital, St. John's wort — reduce area under the curve by 70–90%, risking acute rejection within days. Any interacting drug requires immediate trough monitoring and dose adjustment. Grapefruit juice inhibits intestinal CYP3A4 and should be avoided throughout therapy.

Suggested References

Author / Source	Title	Publication
Katzung BG, ed.	Basic and Clinical Pharmacology, 15th ed. — Chapter on Immunosuppressants	McGraw-Hill; 2021
Brunton L, Knollmann B, Hilal-Dandan R, eds.	Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 14th ed.	McGraw-Hill; 2023
Halloran PF	Immunosuppressive drugs for kidney transplantation	N Engl J Med. 2004;351(26):2715–2729
Vanhove T, Annaert P, Kuypers DRJ	Clinical determinants of calcineurin inhibitor disposition: a mechanistic review	Drug Metab Rev. 2016;48(1):88–112
Staatz CE, Tett SE	Clinical pharmacokinetics and pharmacodynamics of tacrolimus in solid organ transplantation	Clin Pharmacokinet. 2004;43(10):623–653
Naesens M, Kuypers DRJ, Sarwal M	Calcineurin inhibitor nephrotoxicity	Clin J Am Soc Nephrol. 2009;4(2):481–508
Ekberg H, Tedesco-Silva H, Demirbas A, et al	Reduced exposure to calcineurin inhibitors in renal transplantation	N Engl J Med. 2007;357(25):2562–2575
Sehgal SN	Sirolimus: its discovery, biological properties, and mechanism of action	Transplant Proc. 2003;35(3 Suppl):7S–14S
Relling MV, Schwab M, Whirl-Carrillo M, et al	Clinical pharmacogenomics implementation consortium guideline for thiopurine dosing based on TPMT and NUDT15 genotypes	Clin Pharmacol Ther. 2019;105(5):1095–1105
Allison AC, Eugui EM	Mycophenolate mofetil and its mechanisms of action	Immunopharmacology. 2000;47(2–3):85–118
Rhen T, Cidlowski JA	Antiinflammatory action of glucocorticoids — new mechanisms for old drugs	N Engl J Med. 2005;353(16):1711–1723
Kidney Disease: Improving Global Outcomes Transplant Work Group	KDIGO clinical practice guideline for the care of kidney transplant recipients	Am J Transplant. 2009;9(Suppl 3):S1–S155
Haas M, Loupy A, Lefaucheur C, et al	The Banff 2017 kidney meeting report: revised diagnostic criteria for chronic active T cell-mediated rejection and antibody-mediated rejection	Am J Transplant. 2018;18(2):293–307