Camptothecin

Synonym Name: (S)-4-ethyl-4-hydroxy-1H-pyrano[3,4:6,7]indolizino[1,2-b]

quinoline-3,14-(4H,12H)-dione
Molecular formula: C20H16N2O4

Molecular weight: 348.352  
CAS No: 7689-03-4

Camptothecin (CPT) is a cytotoxic quinoline alkaloid which inhibits the DNA enzyme topoisomerase I (topo I). It was discovered in 1966 by M. E. Wall and M. C. Wani in systematic screening of natural products for anticancer drugs. It was isolated from the bark and stem of Camptotheca acuminata (Camptotheca, Happy tree), a tree native in China. CPT showed remarkable anticancer activity in preliminary clinical trials but also low solubility and (high) adverse drug reaction. Because of these disadvantages synthetic and medicinal chemists have developed numerous syntheses of Camptothecin and various derivatives to increase the benefits of the chemical, with good results. Two CPT analogues have been approved and are used in cancer chemotherapy,today, topotecan and irinotecan.

Structures

CPT has a planar pentacyclic ring structure, that includes a pyrrolo[3,4-β]-quinoline moiety (rings A, B and C), conjugated pyridone moiety (ring D) and one chiral center at position 20 within the alpha-hydroxy lactone ring with (S) configuration (the E-ring). Its planar structure is thought to be one of the most important factors in topoisomerase inhibition.

Binding

CPT binds to the topo I and DNA complex (the covalent complex) resulting in a ternary complex, and thereby stabilizing it. This prevents DNA re-ligation and therefore causes DNA damage which results in apoptosis. CPT binds both to the enzyme and DNA with hydrogen bonds. The most important part of the structure is the E-ring which interacts from three different positions with the enzyme. The hydroxyl group in position 20 forms hydrogen bond to the side chain on aspartic acid number 533 (Asp533) in the enzyme. It’s critical that the configuration of the chiral carbon is (S) because (R) is inactive. The lactone is bonded with two hydrogen bonds to the amino groups on arginine 364 (Arg364). The D-ring interacts with the +1 cytosine on non-cleaved strand and stabilizes the topo I-DNA covalent complex by forming hydrogen bond. This hydrogen bond is between carbonyl group in position 17 on the D-ring and amino group on the pyrimidine ring of +1 cytosine. Toxicity of CPT is primarily a result of conversion of single-strand breaks into double-strand breaks during the S-phase when the replication fork collides with the cleavage complexes formed by DNA and CPT.

Physical and chemical properties

The lactone ring in CPT is highly susceptible to hydrolysis. The open ring form is inactive and it must therefore by closed to inhibit topo I. The closed form is favored in acidic condition, as it is in many cancer cells microenvironment. CPT is transported in to the cell by passive diffusion. Cellular uptake is favored by lipophilicity, which enhances intracellular accumulation. Lipophilicity makes compounds more stable because of improved lactone partitioning into red blood cells and consequently less hydrolysis of the lactone. CPT has affinity for human serum albumin (HSA), especially the carboxylate form of CPT. Because of that, the equilibrium between the lactone ring and the carboxylate form is driven toward the carboxylate. Reduced drug-HSA interactions could result in improved activity.