5-Fluoro- and 5-chlorocyclophosphamide: synthesis, metabolism, and antitumor activity of the cis and trans isomers.

In seeking analogues of cyclophosphamide (1) having improved antitumor activity by virtue of accelerated formation of the cytotoxic metabolite phosphoramide mustard, cis and trans isomers of 5-fluoro- and 5-chlorocyclophosphamide (9, 10, 11 and 12, respectively) were synthesized by condensation of the appropriate 3-amino-2-halopropan-1-ol (13 or 26) with N,N-bis(2-chloroethyl)phosphoramidic dichloride (14). The metabolism of the halocyclophosphamides by rat liver microsomes was stereoselective; the cis isomers (9 and 11) were poorly metabolized, whereas the trans isomers (10 and 12) were metabolized with efficiency comparable to that of cyclophosphamide. However, there was no evidence that the yield of phosphoramide mustard produced by the trans analogues were significantly greater than that from cyclophosphamide following microsomal 4-hydroxylation. Hence, the halogen substituents did not accelerate beta-elimination of acrolein from the acyclic aldehydo tautomers. As expected, the poorly metabolized cis-5-fluoride (9) had little activity against the ADJ/PC6 tumor in mice. However, the cis-5-chloride (11) was as active as the trans isomer (12) and each had approximately half the therapeutic index of 1. The trans-5-fluoride (10) was much less active, having an ED90 value some 16-fold that of 1.