Patterns of resistance and therapeutic synergism among alkylating agents.

Alkylating anticancer drugs are varied in chemical structure, alkylating moieties, and likely mechanisms of cytotoxic activity for vital normal cells and sensitive tumor cells. This has been objectively documented by numerous examples illustrating: (1) different in vitro and in vivo reaction products; (2) greater than additive, additive, and less than additive cytotoxicity of drug combinations for vital normal cells in the mouse; (3) readily reproducible and often marked therapeutic synergism between a variety of 2-drug combinations of alkylating agents against a wide variety of histologic types of murine tumors, and (4) observed resistance and cross-resistance of a variety of murine tumors, selected for resistance to specific alkylating agents, compatible with recognized chemical and functional differences between these drugs. The most important observations on resistance and cross-resistance reported are: (a) L1210 cells selected for complete resistance to cyclophosphamide (CPA) retain full sensitivity to selected nitrosoureas (BCNU, CCNU, MeCCNU), chlorozotocin), dianhydrogalactitol, and cis-DDPt, while retaining marked but somewhat reduced sensitivity to L-PAM, piperazinedione, and thioTEPA. (B) L1210 cells selected for resistance to BCNU retain full sensitivity to CPA, L-PAM, and dianhydrogalactitol. They show complete cross-resistance to BIC and variable cross-resistance to other selected nitrosoureas and piperazinedione. (c) L1210/L-PAM has incomplete but marked resistance to L-PAM. It is similar to the parent drug-sensitive line (L1210/0) in response to BCNU, CCNU, MeCCNU, and BIC. It is variably (usually moderately) cross-resistant to CPA, chlorozotocin, dianhydrogalactitol, and thioTEPA, but is completely cross-resistant to cis-DDPt. These resistance and cross-resistance patterns, which are consistent with most other biological and chemical principles established with these alkylating agents, may be useful in selecting alkylating drug combinations for inclusion in chemotherapy protocols in man which, on the basis of diverse observations in animal tumor systems, appear to be clearly indicated.

Isolation and mass spectral identification of blood metabolites of cyclophosphamide: evidence for phosphoramide mustard as the biologically active metabolite.

Thin-layer chromatography and mass spectrometry were used to isolate and identify cyclophosphamide metabolites present in blood of mice. Blood was removed 5, 15, 30 and 45 minutes after intraperitoneal administration and extracted with chloroform followed by methanol. Thin-layer chromatography of the two extracts and the residual solid with or without prior methylation, collection of resulting alkylating components, determination of radioactivity and mass spectral analysis, served to identify cyclophosphamide, 4-ketocyclophosphamide, alcophosphamide, N-dechloroethylcyclophosphamide, carboxyphosphamide, phosphoramide mustard and nor-HN2. The absence of detectable levels of 4-hydroxycyclophosphamide or aldophosphamide in the blood of cyclophosphamide-treated mice suggests that cyclophosphamide is converted rapidly in the liver to carboxyphosphamide, 4-ketocyclophosphamide, phosphoramide mustard and nor-HN2. Direct administration of synthetic 4-hydroxycyclophosphamide to mice and extraction of blood with chloroform demonstrated the recovery of this metabolite in vivo. Analysis of extracts of blood from mice treated with phosphoramide mustard indicated the presence of nor-HN2, 3-(2-chloroethyl)-1,3-oxazolidin-2-one and unchanged drug. Consideration of blood levels, cytotoxicity and alkylating activity of metabolites identified, in or inferred from, this study, implicates phosphoramide mustard as a leading condidate for the biologically active form of cyclophosphamide.