The chemoprotection of cells from apoptosis induced by toxins or ionizing radiation could be important for biodefense and in the treatment of acute injuries. We describe a series of small heterocycles, including fused benzothiazoles, benzimidazoles, and related compounds, that abrogate thymocyte apoptosis induced by dexamethasone and gamma-irradiation. To optimize the protective activity of the previously reported pifithrin-alpha (PFT-alpha, 1), various derivatives and analogues of this and the corresponding ring-closed imidazobenzothiazole (IBT, 39) were synthesized. The aromatic analogues of 39 were more protective than 39, while the aromatic analogues of 1 were not active. Compound 19 containing a pyrrolidinyl substituent on the phenyl ring provided potent antiapoptotic activity (EC50 of 1.31 microM compared to 4.16 microM for 1). Modification of aromatic 39 with a pyrrolidinyl para substituent (compound 60) enhanced the activity, lowering the EC50 to 0.35 microM. Also, 60 provided significant protection against gamma-irradiation-induced apoptosis, as expected. Compounds 19 and 60 may be promising for potential clinical development.