Total structure determination of apratoxin A, a potent novel cytotoxin from the marine cyanobacterium Lyngbya majuscula

Apratoxin A (1), a potent cytotoxin with a novel skeleton, has been isolate d from the marine cyanobacterium Lyngbya majuscula Harvey ex Gomont. This c yclodepsipeptide of mixed peptide-polyketide biogenesis bears a thiazoline ring flanked by polyketide portions, one of which possesses an unusual meth ylation pattern. Its gross structure has been elucidated by spectral analys is, including various 2D NMR techniques. The absolute configurations of the amino acid-derived units were determined by chiral HPLC analysis of hydrol ysis products. The relative stereochemistry of the new dihydroxylated fatty acid unit, 3,7-dihydroxy-2,5,8,8-tetramethylnonanoic acid, was elucidated by successful application of the J-based configuration analysis originally developed for acyclic organic compounds using carbon-proton spin-coupling c onstants ((2,3)J(C,H)) and proton-proton spin-coupling constants ((3)J(H,H) ); its absolute stereochemistry was established by Mosher analysis. The con formation of 1 in solution was mimicked by molecular modeling, employing a combination of distance geometry and restrained molecular dynamics. Apratox in A (1) possesses IC50 values for in vitro cytotoxicity against human tumo r cell lines ranging from 0.36 to 0.52 nM; however, it was only marginally active in vivo against a colon tumor and ineffective against a mammary tumo r.