Maytansinol

Epp et al. were able to show that recombination of CarE with either the spiromycin producer S. ambofaciens or S. lividans (exogenously fed spiromycin was necessary) resulted in the formation of the hybrid compound isovaleryl spiromycin. Recently, the acyltransferase encoding gene asm19 from the gene cluster of the ansa-macrolactam maytansine has been identified through sequence analysis and in-frame deletion. Surprisingly, the corresponding enzyme Asm19 attaches the biologically essential ester side chain of the ansamitocins using N-demethyl-4,5-desepoxymaytansinol as its substrate, and not maytansinol, as was generally believed. Thus, this acyl-transferase acts on a much earlier stage of the maytansin biosynthetic pathway than expected.

In principle, the selective expression of late-stage enzymes can be employed for the engineered production of intermediates with desired functionalities suitable for further chemical modification and detailed SAR studies. However, intermediates partially lacking peripheral decorations are often accumulated in lower yields than their fully modified counterparts. Furthermore, omittance of a specific modification in the biosynthetic assembly logicmight interfere with the substrate specificities of downstream core-decorating enzymes, thus not leading to the accumulation of the desired compound, but to partiallymodified intermediates.For instance, the inactivation of the acyltransferase in the ansamitocin pathway of Actinosynnema pretiosum did not lead to the expected maytansinol, but to its N-demethyl-desepoxy analogue.

As also observed for other tubulin-binding anti-cancer agents, this interaction led to arrest in the G₂-M phase of the cell cycle and subsequently to apoptosis. Early clinical trials with 65 were initiated in the 1970s. However, dose-limiting toxicity and lack of response in the majority of patients enrolled in phase II trials led to the discontinuation of 65 as a single agent for anti-cancer therapy. From the early 1990s on, the maytansinoids were re-investigated for their use as toxic payloads for antibodies. 66 was chosen as the starting material for semisynthesis, since it was readily available from fermentation. DM1 68 and other relatedmaytansinoids suitable for conjugation (such as DM4 69) were synthesized from may- tansinol 67, which is obtained by reductive cleavage of 66 at the acyloxy function at C3 with LiAlH(OMe)₃. Re-esterification of maytansinol with several carboxylic acids mediated by DCC/ZnCl₂ led to new derivatives displaying cytotoxic activity in the 10–90 pMrange, which corresponded to an ideal potency for the use as toxic payloads in antibody conjugates.