Structure Determination, Conformational Analysis, Chemical Stability Studies, and Antitumor Evaluation of the Cryptophycins. Isolation of 18 New Analogs from Nostoc sp. Strain GSV 224

Using a modified isolation procedure devoid of methanol, 18 new cyclic cryptophycins have been isolated from Nostoc sp. GSV 224 as minor constituents in addition to cryptophycins-1 (A), -2 (B), -3 (C), and -4 (D). Acyclic cryptophycins are not found, indicating that the previously reported cryptophycins-5 (E methyl ester), -6 (F methyl ester), and -7 (G) are artifacts produced as a consequence of using methanol in the isolation scheme. Seventeen of the new cyclic analogs differ in structure in either one of the two hydroxy acid units, viz. unit A [(5S,6S,7R,8R)-7,8-epoxy-5-hydroxy-6-methyl-8-phenyl-2(E)-octenoic acid for cryptophycin-1 or (5S,6S)-5-hydroxy-6-methyl-8-phenyl-2(E),7(E)-octadienoic acid for cryptophycin-3] and unit D [(2S)2-hydroxy-4-methylvaleric acid], or one of the two amino acid units, viz. unit B [(2R)-2-amino-3-(3-chloro-4-methoxyphenyl)propionic acid] and unit C [(2R)-3-amino-2-methylpropionic acid], found in the cyclic ABCD peptolide. In unit A of cryptophycins-26, -28, -30, and -40, the methyl group on C-6 is missing or the Δ²-double bond is hydrated. In unit B of cryptophycins-16, -17, -23, -31, -43, and -45, the aromatic ring is phenolic and/or possesses two or zero chlorines. In unit C of cryptophycins-21 and -29, the methyl group on C-2 is missing. In unit D of cryptophycins-18, -19, -49, -50, and -54, a different alkyl group (propyl, isopropyl, or sec-butyl) is attached to C-2. Only one of the new analogs, cryptophycin-24, differs in structure for two units by lacking chlorine in unit B and the methyl group in unit C. Revised structures are presented for cryptophycins-5, -6, and -7 and are correlated with cryptophycin-3, the relative stereochemistry of which has been further rigorously established by X-ray crystallography. NOE studies show that the preferred conformations of most cryptophycins in solution differ from the conformation of cryptophycin-3 in the crystal state. Although cryptophycin-1 is relatively stable at pH 7, both in ionic and nonionic media, the ester bond linking units C and D is fairly labile to solvolysis and mild base hydrolysis. Structure—activity relationship studies indicate that the intact macrolide ring, the epoxide group, the chloro and O-methyl groups in unit B, and the methyl group in unit C are needed for the in vivo activity of cryptophycin-1.