Comparison of the Protonation of Isophosphoramide Mustard and Phosphoramide Mustard

Abstract

The alkylating agent isophosphoramide mustard (IPM) spontaneously forms a relatively stable aziridine derivative which can be directly observed using NMR spectroscopy. The protonations of IMP and its aziridine were probed using 1H, 31P, 15N, and 17O NMR spectroscopy. The positions of the 31P, 15N, and 17O resonances of IPM between pH 2 and 10 each exhibit a single monobasic titration curve with the same pKa of 4.31 +/- 0.02. On the basis of a comparison with other compounds and our earlier work with phosphoramide mustard, the NMR results for IPM indicate that protonation occurs at nitrogen and not oxygen. Over this same pH range, each of the 1H, 31P, and 15N resonances of IPM-aziridine also show a single monobasic titration with a pKa of 5.30 +/- 0.09. The magnitude of the change in chemical shifts suggests that the protonation of the IPM-aziridine occurs at the ring nitrogen. Theoretical gas-phase calculations of PM, IPM, and IPM-aziridine suggest O-protonation to be more likely; however, aqueous phase calculations predict the N-protonated forms to be most stable. Furthermore, for PM and IPM-aziridine, which contain nonequivalent nitrogens, the theoretical calculations and experimental data both agree as to which nitrogen undergoes protonation. These results suggest that the IMP-aziridine remains unprotonated under physiological conditions and may, in part, explain the lower alkylating activity of IPM as compared to PM.

Design and Synthesis of Phosphotyrosine Peptidomimetic Prodrugs

“…The overall 31 P NMR chemical shift change of about 1.25 ppm suggests that protonation of 12b occurs on oxygen rather than nitrogen. 26,27 Synthesis of the analogous phosphorodiamidate peptidomimetic prodrugs 16 -18 is outlined in Schemes 5 and 6. Allyl ester 13a was prepared by reaction of p-aminobenzoic acid potassium salt with allyl bromide in DMF.…”

Gas chromatography–electron ionization mass spectrometry and liquid chromatography–electrospray tandem mass spectrometry for determination of impurities in the anti-cancer drug isophosphoramide mustard

“…In contrast, phosphorus amides are more susceptible to X-N cleavage at lower pH values than are carbonyl amides, and at least three possible mechanisms can account for the cleavage [11].…”

“…1. Biochemical studies suggest that IPM does not alkylate as a typical nitrogen mustard but rather as a phosphorodiamidate anion and may cross-link DNA differently from that of classical nitrogen and phosphoramide mustards [5,9,11]. As expected, the toxicities of IPM are different from those of ifosfamide and CPA [2][3][4]12].…”

“…Isophosphoramide mustard (IPM), a metabolite of ifosfamide, is being readied for clinical trials [1][2][3][4][5][6][7][8][9][10][11][12]. Studies with IPM suggest that the agent has improved anticancer activities in standard animal models, when compared to phosphoramide mustard (PM) and other mustards, ifosfamide and cyclophosphamide (CPA) [9,12].…”

Synthesis of [3H,33P]‐phosphoramide and ‐isophosphoramide mustards and metabolites [3H]‐chloroethylaziridine and ‐aziridine for studies of DNA alkylation

“…[1][2][3] In published 1 and unpublished work, we have shown that phosphoramide mustard (PM, 1) and isophosphoramide mustard (IPM, 2) form aziridinyl intermediates which partition between two pathways: alkylation and P-N bond hydrolysis (Schemes 1 and 2). In the absence of a strong nucleophile, P-N bond scission accounts for up to 90% of the observed products.…”