Apratoxin A, a 25-membered cyclodepsipeptide, is expected to be an anticancer agent. However, the α,β-unsaturated modified cysteine residue (moCys) is one of the challenging aspects in the synthesis of an apratoxin family and contains the Michael acceptor that may cause undesirable in vivo toxicity. Consequently, we designed apratoxin A mimetics in which the moCys moiety is replaced with an amino acid. The first-generation mimetics, apratoxins M1–M7, which contain piperidinecarboxylic acid, 4-aminobutanoic acid, 3-aminopropanoic acid, 3-(aminomethyl)benzoic acid, and their N-methyl derivatives, were designed to maintain a three-dimensional (3D) structure similar to that of apratoxin A, which is critical for its cytotoxicity against cancer cell lines. The mimetics were synthesized using solid-phase peptide synthesis and solution-phase macrolactamization. Apratoxin M7, which contains a piperidinecarboxylic acid moiety, exhibited potent cytotoxicity against HCT-116 cells. A 3D structural analysis using distance geometry calculations confirmed that the overall conformations of apratoxin M7 are similar to those of apratoxin A. Further modification of apratoxin M7 was performed to afford the second-generation mimetics, apratoxins M8–M16. Substitution of each amino acid residue in the tripeptide Tyr(Me)–MeAla–MeIle moiety led to the development of the highly potent apratoxin M16 possessing biphenylalanine (Bph) instead of Tyr(Me), which exhibited an IC50 value of 1.1 nM against HCT-116 cells. Moreover, compared to apratoxin A, apratoxin M16 exhibited similarly high level of growth inhibitory activity against various cancer cell lines. The results indicate that apratoxin M16 would induce the potent cytotoxicity by the same mechanism as apratoxin A. Apratoxin M16 could be utilized as a lead compound in the development of anticancer agents and as a chemical tool for chemical biology studies. In this presentation, details of the conformation-based design, synthesis, biological evaluation, and 3D structural analyses of apratoxin A mimetics modified at the α,β-unsaturated thiazoline moiety will be discussed.