Trabectedin

During the 1950s and 1960s, the National Cancer Institute carried out a wide-ranging program of screening plant and marine organism material. As part of that program, extract from the sea squirt Ecteinascidia turbinata was found to have anticancer activity in 1969.[1] Separation and characterization of the active molecules had to wait many years for the development of sufficiently sensitive techniques, and the structure of one of them, Ecteinascidin 743, was determined by KL Rinehart at the University of Illinois in 1984.[2] Rinehart had collected his sea squirts by scuba diving in the reefs of the West Indies.[3] Recently, the biosynthetic pathway responsible for producing the drug has been determined to come from Candidatus Endoecteinascidia frumentensis, a microbial symbiont of the tunicate.[4] The Spanish company PharmaMar licensed the compound from the University of Illinois before 1994 and attempted to farm the sea squirt with limited success.[3] Yields from the sea squirt are extremely low - 1 tonne of animals is needed to isolate 1 gram of trabectedin - and about 5 grams were believed to be needed for a clinical trial[5] so Rinehart asked the Harvard chemist E. J. Corey to search for a synthetic method of preparation. His group developed such a method and published it in 1996.[6] This was later followed by a simpler and more tractable method which was patented by Harvard and subsequently licensed to PharmaMar.[3] The current supply is based on a semisynthetic process developed by PharmaMar starting from safracin B, an antibiotic obtained by fermentation of the bacterium Pseudomonas fluorescens.[7] PharmaMar have entered into an agreement with Johnson & Johnson to market the compound outside Europe.

Trabectedin was first trialed in humans in 1996.

Trabectedin is composed of three tetrahydroisoquinoline moieties, eight rings including one 10-membered heterocyclic ring containing a cysteine residue, and seven chiral centers.

Proposed scheme for the biosynthesis of trabecteden

The biosynthesis of trabectedin in Candidatus Endoecteinascidia frumentensis starts with a fatty acid loading onto the acyl-ligase domain of the EtuA3 module. A cysteine and glycine are then loaded as canonical NRPS amino acids. A tyrosine residue is modified by the enzymes EtuH, EtuM1, and EtuM2 to add a hydroxyl at the meta position of the phenol, and adding two methyl groups at the para-hydroxyl and the meta carbon position. This modified tyrosine reacts with the original substrate via a Pictet-Spengler reaction, where the amine group is converted to an imine by deprotonation, then attacks the free aldehyde to form a carbocation that is quenched by electrons from the methyl-phenol ring. This is done in the EtuA2 T-domain. This reaction is done a second time to yeid a dimer of modified tyrosine residues that have been further cyclized via Pictet-Spengler reaction, yielding a bicyclic ring moiety. The EtuO and EtuF3 enzymes continue to post-translationally modify the molecule, adding several functional groups and making a sulfide bridge between the original cysteine residue and the beta-carbon of the first tyrosine to form ET-583, ET-597, ET-596, and ET-594 which have been previously isolated.[4] A third O-methylated tyrosine is added and cyclized via Pictet-Spangler to yield the final product.[4]

The total synthesis by E.J. Corey [6] used this proposed biosynthesis to guide their synthetic strategy. The synthesis uses such reactions as the Mannich reaction, Pictet-Spengler reaction, the Curtius rearrangement, and chiral rhodium-based diphosphine-catalyzed enantioselective hydrogenation. A separate synthetic process also involved the Ugi reaction to assist in the formation of the pentacyclic core. This reaction was unprecedented for using such a one pot multicomponent reaction in the synthesis of such a complex molecule.

Recently, it has been shown that trabectedin blocks DNA binding of the oncogenic transcription factor FUS-CHOP and reverses the transcriptional program in myxoid liposarcoma. By reversing the genetic program created by this transcription factor, trabectedin promotes differentiation and reverses the oncogenic phenotype in these cells.[16]

Other than transcriptional interference, the mechanism of action of trabectedin is complex and not completely understood. The compound is known to bind and alkylate DNA at the N2 position of guanine. It is known from in vitro work that this binding occurs in the minor groove, spans approximately three to five bpbase pairs and is most efficient with CGG sequences. Additional favorable binding sequences are TGG, AGC, or GGC. Once bound, this reversible covalent adduct bends DNA toward the major groove, interferes directly with activated transcription, poisons the transcription-coupled nucleotide excision repair complex, promotes degradation of RNA polymerase II, and generates DNA double-strand breaks.[16]