The cyclodepsipeptide jasplakinolide (1) (a.k.a. jaspamide), isolated previously from the marine sponge Jaspis splendens, is a unique cytotoxin and molecular probe that operates through stabilization of filamentous actin (F-actin). We have recently disclosed that two analogues of 1, jasplakinolides B (3) and E, were referred to the National Cancer Institute's (NCI) Biological Evaluation Committee and the objective of this study was to re-investigate a Fijian collection of J. splendens in an effort to find jasplakinolide congeners with similar biological properties. The current efforts have afforded six known jasplakinolide analogues (4 -7, 9 -10), two structures requiring revision (8 and 14) and four new congeners of 1 (11 -13, 15) including open chain derivatives and structures with modified β-tyrosine residues. Compounds were evaluated for biological activity in the NCI's 60 cell line screen and in a microfilament disruption assay in both HCT-116 and HeLa cells. These two phenotypic screens provide evidence that each cytotoxic analogue, including jasplakinolide B (3), operates by modification of microfilaments. The new structure jasplakinolide V (13) has also been selected for study by the NCI's Biological Evaluation Committee. In addition, the results of a clonogenic dose response study on jasplakinolide are presented.The unique cyclodepsipeptide jasplakinolide (1) (a.k.a. jaspamide) has been a seed for continuous chemical and biological investigations since its discovery in 1986. 1,2 This mixed polyketide-peptide compound (often referred to as a PKS-NRPS hybrid) 3 was first isolated from Jaspis (syn: Doryplores) splendens 4 (order Astrophorida, family Ancorinidae) sponges collected in Fiji and Palau, and subsequent investigations [5][6][7][8][9][10] have shown that 1 can be obtained from Jaspis sponges collected throughout the Indo-Pacific. We recently disclosed seven new analogues of 1 ,10 and included an account of the 15 additional natural derivatives divided into two frameworks along with a bio-geographical outline of the two sponge orders that are the source of these metabolites. Other marine sponge genera are also the source of related cyclodepsipeptides named the geodiamolides (Geodia, and Cymbastela) [11][12][13][14][15][16] and the seragamides (Suberites). 17 Similar to the jasplakinolide family, the members of these two classes occur in taxonomically distant groups of sponges. The discovery of yet another † Dedicated to Dr. Koji Nakanishi of Columbia University for his pioneering work on bioactive natural products.* Corresponding author. Tel: 831-459-2603. Fax: 831-459-2935. phil@chemistry.ucsc.edu. ‡ Department of Chemistry and Biochemistry § UCSC Chemical Screening Center ⊥ Henry Ford Hospital Supporting Information Available: 1 H, 13 C, gHMQC and gHMBC NMR spectra for compounds 6-15, a sponge picture of 00101, NCI 60 cell line results, and microfilament effects of 10. This material is available free of charge via the Internet at http://pubs.acs.org. The push to understand the biological ...