Doxorubicin Cardiotoxicity
Generates free radicals causing dilated cardiomyopathy; prevent with Dexrazoxane (iron-chelating agent). Cumulative dose monitoring mandatory.
Bleomycin & Busulfan
Pulmonary fibrosis: Bleomycin (G2 phase arrest; free radical DNA breaks) and Busulfan (alkylating agent; hyperpigmentation 'busulfan tan').
Cyclophosphamide Cystitis
Acrolein metabolite causes hemorrhagic cystitis and transitional cell bladder carcinoma; co-administer Mesna (binds acrolein) + aggressive IV hydration.
Calcineurin Inhibitors
Cyclosporine (binds cyclophilin) and Tacrolimus (binds FKBP-12); inhibit calcineurin -> blocks IL-2 transcription; primary toxicity is nephrotoxicity.
Master Chemotherapeutic Agents & Classic Toxicities
Oncology pharmacology is heavily tested through high-yield agent-specific organ toxicities and rescue therapeutics:
| Chemotherapeutic Agent | Mechanism of Action & Cell Cycle Phase | High-Yield Clinical Application | Pathognomonic Organ Toxicity & Rescue Drug |
|---|---|---|---|
| Methotrexate (MTX) | Inhibits dihydrofolate reductase (DHFR) → ↓ dTMP synthesis (S-phase) | Leukemias, lymphomas, choriocarcinoma, rheumatoid arthritis, ectopic pregnancy | Myelosuppression, hepatotoxicity, mucositis; Rescue: Leucovorin (folinic acid) |
| 5-Fluorouracil (5-FU) | Forms complex with folic acid, inhibits thymidylate synthase → ↓ dTMP (S-phase) | Colorectal carcinoma, basal cell carcinoma (topical) | Myelosuppression (potentiated by leucovorin), coronary vasospasm, hand-foot syndrome |
| Cisplatin / Carboplatin | Cross-links DNA strands; non-cell-cycle specific | Testicular, ovarian, bladder, lung carcinoma | Nephrotoxicity and ototoxicity (acoustic nerve damage); Rescue: Amifostine (free-radical scavenger) + IV hydration |
| Vincristine / Vinblastine | Inhibit microtubule polymerization by binding beta-tubulin → mitotic spindle arrest (M-phase) | Hodgkin / non-Hodgkin lymphomas, ALL, Wilms tumor | Vincristine: Peripheral sensory/motor neuropathy and paralytic ileus; Vinblastine: Severe bone marrow suppression ('blast' bone marrow) |
| Paclitaxel / Docetaxel | Hyperstabilize polymerized microtubules, preventing breakdown → arrest in anaphase (M-phase) | Ovarian, breast, and non-small cell lung cancer | Peripheral neuropathy, neutropenia, hypersensitivity (premedicate with dexamethasone and diphenhydramine) |
Transplant & Autoimmune Immunosuppressants
Preventing organ allograft rejection requires multi-drug regimens targeting distinct stages of T-cell and B-cell activation:
| Immunosuppressive Drug | Molecular Target | Mechanism of Immunosuppression | Key Board Toxicity / Differentiator |
|---|---|---|---|
| Cyclosporine | Cyclophilin | Blocks calcineurin → prevents NFAT dephosphorylation → ↓ IL-2 transcription | Nephrotoxicity (afferent arteriolar vasoconstriction), hypertension, gingival hyperplasia, hirsutism |
| Tacrolimus (FK506) | FKBP-12 | Blocks calcineurin → prevents NFAT dephosphorylation → ↓ IL-2 transcription | Nephrotoxicity, neurotoxicity (tremor, seizures), hyperglycemia/diabetes; no gingival hyperplasia |
| Sirolimus (Rapamycin) | FKBP-12 / mTOR | Inhibits mTOR kinase → blocks IL-2 signal transduction and cell cycle progression | Pancytopenia, severe hyperlipidemia, insulin resistance; NOT nephrotoxic ('kidney-sparing') |
| Mycophenolate Mofetil | Inosine Monophosphate Dehydrogenase (IMPDH) | Reversibly inhibits IMPDH → blocks de novo purine synthesis in B and T lymphocytes | GI distress (nausea, diarrhea), pancytopenia; associated with CMV infection |
| Azathioprine / 6-MP | HGPRT / Purine synthesis | Prodrug converted to 6-mercaptopurine; blocks de novo purine nucleotide synthesis | Pancytopenia; Fatal toxicity when co-administered with Allopurinol (blocks xanthine oxidase) |
Board Traps & Common Distractors
- Azathioprine or 6-Mercaptopurine dose MUST be reduced by 75% if the patient is concurrently taking Allopurinol or Febuxostat; xanthine oxidase metabolizes 6-MP, and its inhibition causes lethal pancytopenia.
- Cyclophosphamide and ifosfamide produce acrolein in the urine, causing life-threatening hemorrhagic cystitis; Mesna and aggressive hydration must always be co-administered.
- Dexrazoxane must be administered with high-dose Doxorubicin to prevent irreversible dilated cardiomyopathy caused by iron-mediated hydroxyl radical peroxidation.