Eighteen configurational isomers of the
antimitotic peptide dolastatin 10 (Bai et al., Biochem Pharmacol 39: 1941-1949, 1990) derived from Dolabella auricularia, together with segments obtained as precursors in its synthesis (Pettit et al., J Am Chem Soc 111: 5463-5465, 1989), were examined as inhibitors of
tubulin polymerization and as inhibitors of growth of L1210 murine
leukemia cells in culture.
Dolastatin 10 consists of four
amino acids (in order from the amino terminus:
dolavaline,
valine,
dolaisoleucine, and
dolaproine), three unique to D. auricularia, linked to an unusual primary
amine (
dolaphenine, probably derived from
phenylalanine) at what would otherwise be its carboxyl terminus.
Dolastatin 10 has nine asymmetric
carbon atoms, and available isomers included alternate configurations at five positions (positions 9 and 10 in the
dolaproine moiety and positions 18, 19 and 19a in the
dolaisoleucine moiety). For
tubulin polymerization, only alterations at positions 18 and 19 resulted in loss of inhibitory activity of the isomer. In addition, a tripeptide containing
dolavaline,
valine and
dolaisoleucine with all asymmetric carbons identical configurationally to those in
dolastatin 10 was found to be about 30% as effective as
dolastatin 10 in inhibiting
tubulin polymerization. Cytotoxic effects were much more sensitive to alterations in the
dolastatin 10 structure. The only modification which did not lead to reduced cytotoxicity was reversal of configuration at position 19a in the
dolaisoleucine moiety. Both this isomer and
dolastatin 10 had IC50 values of less than 1 nM. Several other isomers had IC50 values with the L1210 cells in the range of 30-90 nM, but these did not correlate well with their inhibitory effects on
tubulin polymerization. The tripeptide effective as an inhibitor of
tubulin polymerization had no activity against the L1210 cells.