Camptothecin

Camptothecin (CPT) is a cytotoxic quinoline alkaloid of the monoterpene indole class, first isolated from the bark and wood of Camptotheca acuminata, a tree native to southern China. It acts on topoisomerase I (Top1), the enzyme that relieves torsional stress by transiently cleaving one DNA strand and allowing the broken strand to rotate around the intact strand before resealing it. Camptothecin binds at the interface of the transient Top1–DNA cleavage complex and blocks the resealing step, so the enzyme remains covalently trapped on nicked DNA. The trapped complex is itself reversible and drug removal releases it; the lethal event comes when an advancing replication fork or transcription complex collides with it, converting a repairable nick into a double-strand break. That requirement makes the drug S-phase specific, and makes exposure duration matter more than peak concentration.

Its chemistry is the practical constraint. The E-ring α-hydroxylactone required for activity hydrolyzes at physiological pH to an inactive carboxylate, an equilibrium that human serum albumin pushes further toward the open form, so the parent compound loses potency in plasma while remaining poorly soluble and toxic. Semisynthetic derivatives address this: irinotecan hydrochloride is a prodrug converted by carboxylesterases to the far more potent SN-38, whose clearance depends on UGT1A1-mediated glucuronidation and whose toxicity therefore tracks UGT1A1 genotype; topotecan is used directly; and exatecan remains investigational as a free drug.

Precise delivery has given the scaffold a second life. Camptothecin analogs are potent at picomolar concentrations, which is what an antibody-drug conjugate needs from a payload, and the most successful current conjugates carry either SN-38 or an exatecan derivative on a cleavable linker — an arrangement that concentrates the drug at the tumor and exploits its membrane permeability to kill neighboring antigen-negative cells. Prodrug design, linker chemistry, and nanoparticle formulation continue to be pursued for the same reason: the molecule kills reliably, and nearly every difficulty with it concerns getting an intact lactone to the right place.