Chemistry of the Immunomodulatory Macrolide Ascomycin and Related Analogues

Abstract

Ascomycin and its close homologue tacrolimus are highly complex macrocyclic natural compounds isolated from the fermentation broths of Streptomyces hygroscopicus var. ascomyceticus and Streptomyces tsukubaensis. They are potent T-cell inhibitors of the phosphatase calcineurin, as is cyclosporin A, which, apart from its therapeutic value to inhibit transplant rejection, was found in the 1980s to be highly efficacious after oral application in the treatment of inflammatory skin diseases. However, efforts failed in rendering this calcineurin inhibitor as a topical agent to minimize its systemic side effects. Using a new and human-related skin inflammation model, calcineurin inhibitors of the ascomycin- and tacrolimus-type were shown to be highly effective after topical application and thus provided the first pharmacological evidence of a novel class of anti-inflammatory agents for topical treatment. The following extensive medicinal chemistry efforts finally resulted in the selection of the ascomycin derivative pimecrolimus for development, due to its favorable pharmacological and safety profiles. Since 2001/2, the new class of topical calcineurin inhibitors, represented by pimecrolimus cream and tacrolimus ointment, has become the first and only alternative to topical corticosteroids for the treatment of the inflammatory skin diseases atopic dermatitis. The presently available biological properties and clinical data of pimecrolimus are summarized. Medicinal chemistry efforts aimed at an understanding of the structure-activity relationships required a detailed understanding of the chemical properties of the highly complex macrolactam structure of ascomycin and its derivatives, and elaboration and establishment of methodologies to selectively protect, modify and transform the structural elements. This chapter summarizes for the first time the extensive chemical efforts from our laboratories and also discusses the work published by others in this field. Owing to the complex structure in the macrophilin binding tricarbonyl domain, the active compounds exist as mixtures of several isomers. The potential equilibrium products including “furano-ascomycins” have been synthesized. The tricarbonyl moiety is highly sensitive and undergoes a benzilic acid-type of rearrangement, and this has been elaborated for synthesizing labeled ascomycins. Protocols have been developed resulting in selective reactions of diazomethane with tacrolimus. A new class of derivatives termed “cyclo-ascomycins” arising through cyclization in the binding domain has been synthesized. In addition to the synthesis of various other derivatives through selective reactions in the tricarbonyl region, photochemical investigations have led to novel modifications on the pipecolic acid unit. Protocols have been developed for cleavage of the C-1−C-9 unit comprising pipecolic acid and modification of the fragment to new analogues featuring new amino acids in place of pipecolic acid. In the effector side of the molecule, analogues have been prepared through allylic oxidation at C-18, epimerization of C-21, modification of the C-21−allyl side chain through Grubbs’ cross metathesis reactions, and dehydration of the C-24−OH followed by selective addition reactions to the resulting enone. On the cyclohexyl part of the molecule, demethylation, introduction of additional hydroxy groups, extensive derivatization of the C-33−OH, and ring contractions have been achieved. The ring-contracted derivative, SDZ 281–240, was the first topical calcineurin inhibitor to demonstrate clinical proof of concept in patients with an inflammatory skin disease. Furthermore, the cyclohexyl-methylidene group was replaced successfully with several moieties. The synthesis of iso-ascomycin, a ring contracted derivative, and its further chemistry are summarized. In conclusion, the chapter summarizes the extensive chemistry and biology studies on a natural product, which have resulted in a novel therapy approved worldwide. This underscores the importance of natural products as a versatile source of novel structures with unique biological acitivities.