Antimetabolites

Mechanism and Class Overview

• Class mechanism: Most antimetabolites act in S phase, but cladribine is cytotoxic to both resting and dividing lymphocytes. They mimic physiologic metabolites (folate, pyrimidines, purines) and either inhibit the enzymes of nucleotide synthesis or are incorporated into DNA/RNA to block replication and repair.
• Resistance: ↓ drug transport into the cell (RFC, FRP), ↑ target enzyme (DHFR, thymidylate synthase), ↑ catabolic enzymes, and ↑ salvage-pathway activity; loss of MMR enzymes for several agents.

Antifolates

• Class handling: Antifolates resemble physiologic folate, are transported in by the reduced folate carrier (RFC) and folate receptor protein (FRP), and are polyglutamated intracellularly by FPGS. Resistance arises from ↓ RFC, ↓ FRP, or ↑ DHFR.

Methotrexate (MTX)

• Mechanism: Inhibits dihydrofolate reductase (DHFR), which converts dihydrofolate to the active tetrahydrofolate; MTX affinity for DHFR is roughly 1000-fold that of folate, so dTMP and de novo purine synthesis fall.
• Pharmacokinetics: Low oral bioavailability; renally eliminated. Renal elimination is inhibited by ASA, NSAIDs, penicillins, cephalosporins, probenecid, PPIs, and TMP-SMX (avoid around treatment); dose reduce in renal dysfunction. Half-life is prolonged by third-space fluid (pleural effusion, ascites), so tap and obtain a CXR before treatment.
• High-dose MTX (HDMTX): Aggressive hydration and urine alkalinization starting 12 h before, continued until MTX falls below the safe level (about <0.05 to 0.1 micromolar) rather than a fixed 24 to 48 h; extend when clearance is delayed, alkalinize urine to pH > 7.0 with NaHCO3, check MTX level q24h until < 5 × 10^-8 M, and give leucovorin rescue starting 24 h after the dose. Use mtxpk.org to model delayed clearance.
• Leucovorin rescue: Leucovorin (folinic acid) is converted to reduced folates (tetrahydrofolate) and does not require DHFR for activation, so it bypasses the MTX blockade and restores purine/pyrimidine synthesis in normal cells, ↓ toxicity (especially myelosuppression). It acts intracellularly. Timing matters: given too early it can also blunt antitumor activity, so it is given 24 h after MTX.
• Glucarpidase: For AKI with markedly elevated MTX, give within 48 to 60 h; it cleaves extracellular MTX and drops the level by ≥ 97% within 15 min. It works only on renal/plasma (extracellular) MTX. Do not give leucovorin within 2 h before or after glucarpidase (leucovorin is also a glucarpidase substrate); continue leucovorin otherwise.
• Toxicities: Myelosuppression, mucositis (3 to 7 d post-dose), nephrotoxicity (MTX crystal precipitation in acidic tubules with low urine flow), hepatotoxicity, pneumonitis, and HDMTX neurotoxicity (acute, subacute, chronic).
• Leukoencephalopathy: HDMTX neurotoxicity can manifest as toxic leukoencephalopathy (headache, confusion, seizures, PRES, cerebellar dysfunction). MRI shows diffuse T2/FLAIR hyperintensity in deep periventricular white matter and the corpus callosum, sparing basal ganglia and subcortical U fibers. There is no definitive treatment: stop the offending agent and give supportive care (corticosteroids, antioxidants). 5-FU, which crosses the blood-brain barrier, can cause the same picture.
• Risk factors for toxicity: Older age, drug interactions, low albumin, renal dysfunction, low folate, and third-spacing.

Pemetrexed (Alimta)

• Mechanism: Multi-targeted antifolate; primarily inhibits thymidylate synthase (blocking dTMP synthesis) and also DHFR and GAR-FT.
• Indications: Non-squamous NSCLC, mesothelioma.
• Toxicities: Myelosuppression, diarrhea, fatigue. Low folate and high homocysteine (> 10) predict worse toxicity, so folate and B12 supplementation is required. Skin rash (dexamethasone 4 mg BID prophylaxis the day before, of, and after). Transient LFT elevation. Hold if CrCl < 45.

Pralatrexate (Folotyn)

• Mechanism: Antifolate designed for high affinity to the RFC-1 transporter and to be a better FPGS substrate.
• Indications: Relapsed/refractory peripheral T-cell lymphoma. Give folate and B12.
• Toxicities: Mucositis, myelosuppression, anorexia, fatigue, hepatotoxicity.

Fluoropyrimidines

5-Fluorouracil (5-FU)

• Mechanism: 5-FU is activated to FdUMP, which inhibits thymidylate synthase (TS). FdUMP forms a ternary complex with TS and 5,10-methylene-THF (derived from leucovorin); leucovorin stabilizes this complex and keeps TS inhibited, so leucovorin enhances 5-FU activity (opposite of its rescue role with MTX). Other metabolites are incorporated into DNA (FdUTP, dUTP) and RNA (FUTP).
• Indications: Colorectal, breast, and other GI cancers.
• DPD metabolism: Metabolized ~80% by the liver via dihydropyrimidine dehydrogenase (DPD), which is present in WBCs, GI mucosa, and liver (hence the very short 5-FU half-life of 5 to 20 min and poor oral bioavailability). DPD deficiency (3 to 5% of patients) causes severe 5-FU toxicity (GI, myelosuppression, neurotoxicity). FDA boxed warning (Feb 2026): test for DPYD variants before starting 5-FU or capecitabine unless treatment is urgent; avoid in complete DPD deficiency.
• Toxicities: Myelosuppression (greater with bolus); mucositis, diarrhea, and hand-foot syndrome (greater with infusion). Cardiotoxicity: coronary vasospasm (chest pain) in roughly 1 to 5% (large cohort 2.16%), typically chest pain within 12 h of the first dose (may present as arrhythmia or pulmonary edema); work up with EKG. Can cause tear-duct stenosis and can cross the blood-brain barrier (leukoencephalopathy). Rescue is uridine triacetate for 5-FU/capecitabine overdose or early-onset severe/life-threatening toxicity; start as soon as possible and within 96 h of ending administration (thymidine no longer available).

Capecitabine (Xeloda)

• Mechanism: Oral prodrug of 5-FU with good absorption (bypasses gut DPD degradation); converted to 5-FU in three enzymatic steps (two in liver via cytidine deaminase and carboxylesterase, the final step inside the tumor cell by thymidine phosphorylase).
• Indications: Colorectal and breast cancers (oral alternative to infusional 5-FU).
• Toxicities: Resemble continuous-infusion 5-FU: more hand-foot syndrome and diarrhea, less myelosuppression and mucositis than bolus 5-FU, mild hyperbilirubinemia. DPD deficiency again predicts severe toxicity. Interacts with warfarin (↑ INR/bleeding) and phenytoin (↑ phenytoin toxicity). Dose reduce if CrCl 30 to 50, avoid if CrCl < 30.

Trifluridine/tipiracil (TAS-102, Lonsurf)

• Mechanism: Trifluridine plus tipiracil (tipiracil blocks trifluridine degradation). Trifluridine monophosphate inhibits TS and the triphosphate is incorporated into DNA to ↓ DNA synthesis.
• Indications: Refractory metastatic colorectal cancer, effective in 5-FU-resistant disease regardless of KRAS status (RECOURSE); now given with bevacizumab (SUNLIGHT, OS benefit, FDA Aug 2023). Also refractory gastric/GEJ adenocarcinoma (TAGS).
• Toxicities: Myelosuppression (main), GI, fatigue, anorexia.

Cytidine Analogs

Cytarabine (Ara-C)

• Mechanism: Parent ara-C is converted to active ara-CTP, which inhibits DNA polymerase alpha (↓ synthesis) and beta (↓ repair), incorporates into DNA (ribonucleotide reductase inhibition is a gemcitabine feature, not cytarabine). Deactivating enzymes are abundant in gut and blood (hence poor oral bioavailability and short 5 to 20 min half-life, so conventional AML induction uses continuous IV infusion (high-dose cytarabine is intermittent; also given SC and intrathecally)).
• Indications: AML, ALL, lymphoma; high-dose (HIDAC) for CNS penetration.
• Toxicities: Myelosuppression, GI, cholestasis and pancreatitis. Ara-C syndrome (fever, myalgia, arthralgia, rash, conjunctivitis, seen in children). HIDAC neurotoxicity: cerebral and cerebellar dysfunction, confusion, ataxia, coma, symptoms starting several days into treatment; stop cytarabine promptly, recovery is variable and may be incomplete or permanent. Conjunctivitis is prevented with prophylactic corticosteroid eye drops started the day before.

Gemcitabine (Gemzar)

• Mechanism: Gemcitabine diphosphate inhibits ribonucleotide reductase (depletes dNTPs, self-potentiation); the triphosphate is incorporated into DNA (masked chain termination). ~90% cleared in urine within 24 h; clearance is ~30% lower in women and the elderly.
• Indications: Pancreatic, bladder, breast, and lung cancers; lymphoma.
• Toxicities: Myelosuppression, nausea/vomiting, flu-like symptoms, mild LFT elevation; rare pneumonitis/ARDS and hemolytic-uremic syndrome (< 1%; gemcitabine is a classic dose-related cause of drug-induced HUS along with mitomycin C and cyclosporine).

Thiopurines

6-Mercaptopurine (6-MP)

• Mechanism: Purine analog (also the active moiety of the prodrug azathioprine); the monophosphate inhibits de novo purine synthesis and the triphosphate is incorporated into DNA/RNA to ↓ their synthesis. Resistance from ↑ TPMT and ↑ xanthine oxidase (XO) activity and loss of MMR (hMLH1, hMSH2).
• Indications: ALL (maintenance), AML.
• Drug interactions: 6-MP is degraded by xanthine oxidase; allopurinol inhibits XO and raises 6-MP levels/toxicity (reduce 6-MP dose). TPMT deficiency also raises levels/toxicity.
• Toxicities: Myelosuppression, GI/mucositis, hepatotoxicity, immunosuppression (↓ cell-mediated immunity, ↑ bacterial/fungal/parasitic infection), secondary malignancies.

6-Thioguanine (6-TG)

• Mechanism: Purine analog incorporated into DNA. Unlike 6-MP it is not a xanthine oxidase substrate, so allopurinol does not require a 6-TG dose adjustment; TPMT deficiency still increases toxicity.
• Indications: ALL, AML.
• Toxicities: Myelosuppression, hepatotoxicity (hepatic veno-occlusive disease), secondary malignancies.

Purine Analogs

Fludarabine (Fludara)

• Mechanism: Active fludarabine triphosphate is incorporated into DNA, ↓ DNA synthesis and repair.
• Indications: CLL, SLL, lymphoma; also lymphodepleting conditioning before cellular therapy, including TIL/TCR products for solid tumors (lifileucel, afami-cel). Dose adjust in renal impairment (renal clearance ~40% of total; reduce for CrCl 30 to 79, avoid if CrCl < 30).
• Toxicities: Marked immunosuppression (↓ T-cell activity, PJP risk, so give PJP and antiviral prophylaxis; CD4 recovery can take ~1 year), tumor lysis syndrome, fever, myelosuppression with slow count recovery, GI.

Cladribine (Leustatin)

• Mechanism: Adenosine nucleoside analog resistant to adenosine deaminase, so it accumulates intracellularly, incorporates into DNA, and triggers apoptosis (especially in B and T lymphocytes).
• Indications: Hairy cell leukemia, CLL.
• Toxicities: Three boxed warnings: nephrotoxicity (dose adjust CrCl < 50), neurotoxicity (crosses the blood-brain barrier, paresis), and prolonged myelosuppression. No live vaccines.

Pentostatin (Nipent)

• Mechanism: Adenosine deaminase inhibitor.
• Indications: Hairy cell leukemia, CLL, cutaneous T-cell lymphoma.
• Toxicities: Myelosuppression, immunosuppression, hepatotoxicity, neurotoxicity.

Ribonucleotide Reductase Inhibitor

Hydroxyurea

• Mechanism: Inhibits ribonucleotide reductase, depleting deoxyribonucleotide pools.
• Indications: Sickle cell disease, CML, polycythemia vera, essential thrombocythemia, cytoreduction in hyperleukocytosis.
• Toxicities: Myelosuppression, macrocytic anemia, mucocutaneous ulcers, secondary malignancies.

Hypomethylating Agents

• Class mechanism: Incorporated into DNA and inhibit (trap) DNA methyltransferases, reducing DNA methylation; azacitidine also incorporates into RNA (predominantly), decitabine into DNA only.

Azacitidine

• Dosing: Given IV/SQ x7 d q28d for AML/MDS; dose adjust for rising Cr, LFT elevation, or bicarbonate < 20.
• Toxicities: Myelosuppression, nausea/vomiting, fever, chills, rigors.

Decitabine

• Dosing: Given IV x5 d q28d for AML/MDS; not studied in pre-existing renal/hepatic impairment (no validated starting-dose adjustment); label directs delaying subsequent cycles for serum creatinine ≥2 mg/dL or ALT/total bilirubin ≥2x ULN until resolved.
• Toxicities: Myelosuppression, nausea/vomiting/diarrhea, fever, hypokalemia, hypomagnesemia, hyperglycemia, hyperbilirubinemia.