The growth and metastasis of prostate cancer are regulated by prostate stroma through the tumor-stromal cell interactions. Small molecules that modulate the tumor-stromal cell interactions will be new anticancer drugs. In the course of our screening of the modulators, we isolated two new atpenins, NBRI23477 A (4) and B (5), from the fermentation broth of Penicillium atramentosum PF1420. Compounds 4 and 5 as well as atpenin A4 (1), A5 (2) and B (3) inhibited the growth of human prostate cancer DU-145 cells in the coculture with human prostate stromal cells more strongly than that of DU-145 cells alone.