Phosmidosine B

This paper describes the first synthesis of phosmidosine and phosmidosine B, i.e., nucleotide antibiotics composed of 8-oxoadenosine and L-proline which are connected via a unique N-acyl phosphoramidate linkage. Phosmidosine has a yet-undetermined chiral center at the phosphorus atom of the N-acyl phosphoramidate linkage. Phosmidosine B is a demethylated phosmidosine derivative with no chirality on the phosphorus. Phosmidosine B was successfully synthesized by the reaction of an N-acetyl-8-oxoadenosine 5'-O-phosphoramidite derivative with an N-protected prolinamide in the presence of 5-(3,5-dinitrophenyl)-1H-tetrazole. The successful synthesis of phosmidosine was achieved by use of a tert-butoxycarbonyl (Boc) group, which was found to be selectively introduced into the 7-NH function of 8-oxoadenosine and to serve as a pseudo-protecting group due to its steric effect in such manner that the unmasked 6-amino group was not phosphitylated. Final coupling reaction of the 8-oxoadenosine 5'-phosphoramidite derivative with N-tritylprolinamide followed by full deprotection gave a mixture of phosmidosine and its diastereoisomer. The (13)C NMR spectra of the diastereomers suggest that the slow-eluted diastereomer 1b is the naturally occurring phosmidosine. The growth inhibitory activity of phosmidosine 1b, its diastereomer 1a, and phosmidosine B in various tumor cell lines was evaluated by the MTT assay. As the result, phosmidosine B showed high anticancer activities and both the diastereomers 1a and 1b of phosmidosine isolated were found to have similar but approximately 10 times higher anticancer activities than phosmidosine B. Moreover, it turned out that these phosmidosine derivatives showed characteristic inhibitory activities against cancer cells independent of their p53 phenotypes. These results suggest that phosmidosine and its related compounds would be promising as a new type of anticancer agents having a wide range of inhibitory activities against tumor cells.

An antifungal antibiotic, phosmidosine (1) was previously isolated (J. Antibiotics 44: 375 approximately 381, 1991). Phosmidosine derivatives, phosmidosines B (2) and C (3) were newly isolated as detransforming compounds from the fermentation broth of Streptomyces sp. strain RK-16 which is a producer strain of phosmidosine (1). The structures of 2 and 3 were established by spectroscopic methods, including UV, HRFAB-MS, and NMR. 1 and 2 showed the inhibitory activity of the cell cycle progression and the morphological reversion activity on srcts-NRK cells. On the other hand, 3 had no activity. These results indicate that the prolyl group in phosmidosine derivatives plays an important role in the inhibitory activity against the cell cycle progression and the morphological reversion activity on srcts-NRK cells.