Robert F. Struck

Modulation of cyclophosphamide activity by O 6-alkylguanine-DNA alkyltransferase

Purpose: The human medulloblastoma cell line D283 Med (4-HCR), a line resistant to 4-hydroperoxycyclophosphamide (4-HC), displays enhanced␣repair of DNA interstrand crosslinks induced by phosphoramide mustard. D283 Med (4-HCR) cells are cross-resistant to 1,3-bis(2-chloroethyl)-1-nitrosourea, but partial sensitivity is restored after elevated levels of O6-alkylguanine-DNA alkyltransferase (AGT) are depleted by O6-benzylguanine (O6-BG). Studies were conducted to define the activity of 4-HC and 4-hydroperoxydidechlorocyclophosphamide against D283 Med (4-HCR) after AGT is depleted by O6-BG. Methods: Limiting dilution and xenograft studies were conducted to define the activity of 4-HC and 4-hydroperoxydidechlorocyclophosphamide with or without O6-BG. Results: The activity of 4-HC and 4-hydroperoxydidechlorocyclophosphamide against D283 Med (4-HCR) was increased after AGT depletion by O6-BG preincubation. Similar studies with Chinese hamster ovary cells, with or without stable transfection with a plasmid expressing the human AGT protein, revealed that the AGT-expressing cells were significantly less sensitive to 4-HC and 4-hydroperoxydidechlorocyclophosphamide. Reaction of DNA with 4-HC, phosphoramide mustard, or acrolein revealed that only 4-HC and acrolein caused a decrease in AGT levels. Conclusions: We propose that a small but potentially significant part of the cellular toxicity of cyclophosphamide in these cells is due to acrolein, and that this toxicity is abrogated by removal of the acrolein adduct from DNA by AGT.

Denaturation of cytochrome P-450 by cyclophosphamide metabolites.

Abstract Cyclophosphamide (CP) metabolites, acrolein and 4-hydroxy-CP, were found to denature rat liver microsomal cytochrome P-450, whereas another metabolite, phosphoramide mustard, CP per se or its analog Ifosfamide had no effect. The denaturation produced by CP metabolites could be blocked by cysteine, suggesting an interaction between CP metabolite(s) and sulfhydryl groups in cysteine and probably in cytochrome P-450. These studies might explain the biochemical basis of the specific depression of various microsomal mixed function oxygenase activities produced by high doses of CP.

Comparative in vitro cytotoxicity of cyclophosphamide, its major active metabolites and the new oxazaphosphorine ASTA Z 7557 (INN mafosfamide)

SummaryCyclophosphamide (CPA), the most commonly used alkylating agent in the treatment of a wide variety of hematologic and solid tumors, requires oxidation by hepatic microsomal enzymes to its active alkylating species. A number of alternative methods exist to simulate the in vitro cytotoxicity of CPA against animal and human tumors, including the co-incubation of CPA with the S-9 fraction of rat liver homogenates (S-9) and the use of either 4-hydroperoxy CPA (a stabilized form of a major blood-borne metabolite of CPA), phosphoramide mustard (PM, considered to be the ultimate intracellular alkylating metabolite of CPA), or ASTA Z 7557 [4-(2-sulfonatoethylthio)-CPA, a new oxazaphosphorine compound which after dissolution undergoes rapid spontaneous hydrolysis in vitro with liberation of 4-hydroxy-CPA]. Using a human tumor clonogenic assay (HTCA) we have quantitated the median molar inhibitory dose 50 (ID50) concentrations of S-9 activated-CPA, 4-hydroperoxy-CPA, PM, and ASTA Z 7557 against 107 previously untreated tumors, as well as determining the in vitro biological stability of the former three CPA metabolite preparations. 4-Hydroperoxy-CPA proved the most consistently cytotoxic (median molar ID50=5.77#x00D7;10−5M) compound, followed by ASTA Z 7557, S-9 activated-CPA and PM in that order. Of additional interest S-9 activated CPA and PM proved relatively unstable biologically when frozen at -120°C, whereas 4-hydroperoxy-CPA lost none of its cytotoxicity over a 36 day period during freezing. On the basis of these data 4-hydroperoxy-CPA appears the compound of choice for use in vitro to evaluate the activity that CPA is likely to express clinically against solid tumors. Since 4-hydroperoxy-CPA is not available for clinical use, ASTA Z 7557, which was slightly less cytotoxic to ovarian cancers and a wide variety of other tumors in the HTCA, appears an attractive agent to develop further clinically, especially for regional chemotherapy (e.g., intraperitoneal and intra-arterial treatment) of solid tumors.

Inhibition of NADPH-cytochrome P450 reductase by cyclophosphamide and its metabolites

Abstract Cyclophosphamide (CP) administration to rats produced a dose-dependent loss of hepatic NADPH-cytochrome-P450 reductase and microsomal mixed function oxidase (MFO) activities. In vitro CP, its metabolites (acrolein, phosphoramide mustard, 4-keto CP and nor-nitrogen mustard) and Ifosfamide, which is an analog of CP, were tested for their effects on the reductase activity. Only acrolein produced a significant loss of the reductase (66%). This loss of activity could be prevented by the presence of cysteine in the incubation mixture. Acrolein also produced a dose dependent loss of the activity when incubated with the purified reductase. These data suggest that CP-induced loss of the reductase results from interaction between CP metabolite acrolein and critical sulfhydryl groups in the reductase.

The effect of "activated" cyclophosphamide on rat granulosa cells in vitro

We investigated the mechanism of cyclophosphamide (CTX)-induced ovarian toxicity by studying the effect of an activated form, 4-hydroperoxycyclophosphamide (PCTX), on rat granulosa cells in vitro. Cells were obtained from PMSG-primed immature rats and incubated with PCTX at concentrations of 1, 10, 100, and 500 micrograms/mL. Ovine LH (10 ng/mL) was added in selected tubes. Cell viability before and after seven hours incubation was determined. Progesterone and prostaglandin E accumulation were measured by radioimmunoassay. Granulosa cell viability was significantly decreased at PCTX concentrations of 10 micrograms/mL or higher in a dose-related manner. PCTX at concentrations of 100 micrograms/mL and 500 micrograms/mL significantly decreased basal and LH-induced progesterone and prostaglandin E accumulation. The above findings demonstrate that cyclophosphamide metabolites decrease granulosa cell survival and function in vitro. These direct effects suggest a possible mechanism for CTX-induced premature ovarian failure.

The Relationship of the Metabolism of Anticancer Agents to Their Activity

‘When I use a word,’ Humpty Dumpty said in a rather scornful tone, ‘it means just what I choose it to mean — neither more nor less.’[1]

Antitumor activity of halogen analogs of phosphoramide, isophosphoramide, and triphosphoramide mustards, the cytotoxic metabolites of cyclophosphamide, ifosfamide, and trofosfamide.

A series of halogen analogs of phosphoramide mustard, isophosphoramide mustard, and triphosphoramide mustard, the cytotoxic metabolites of the antitumor drugs cyclophosphamide, ifosfamide, and trofosfamide, respectively, was evaluated in vitro against human tumor cell lines and in vivo against experimental tumors to investigate the effect of replacement of chlorine with bromine or fluorine on the antitumor activity of the parent phosphoramide mustards. In the experimental tumors L1210 leukemia, B16 melanoma, mammary adenocarcinoma 16/C, and ovarian sarcoma M5076, the antitumor activity of the analogs was observed to be generally comparable with that of the parent mustards when chlorine was replaced by bromine but uniformly lower when chlorine was replaced by fluorine. Furthermore, the monobromo analog of isophosphoramide mustard displayed equal or somewhat greater activity in comparison with cyclophosphamide when evaluated against subcutaneously implanted L1210 leukemia with intraperitoneal drug treatment and against mammary adenocarcinoma 16/C.

Identification of metabolites of 9-β-d-arabinofuranosyl-2-fluoroadenine, an antitumor and antiviral agent

Analysis of blood from a dog given a 400 mg/m2 dose of 9-beta-D-arabinofuranosyl-2-fluoroadenine (2-F-araA) led to the identification of parent drug and a major metabolite, 9-beta-D-arabinofuranosyl-2-fluorohypoxanthine. 2-Fluoroadenine, a toxic derivative of 2-F-araA, was not detected in blood within the limits of detection, suggesting that parent drug was absorbed and distributed without systemic exposure to this toxic derivative. Parent drug, 2-fluoroadenine, and 9-beta-D-arabinofuranosyl-2-fluorohypoxanthine were identified in urine of dog, monkey, and mouse.

Determination of the phamacophore of penclomedine, a clinically-evaluated antitumor pyridine derivative

The main objective of this investigation was to identify the reactive pharmacophore in penclomedine (PEN, 3,5-dichloro-4,6-dimethoxy-2-(trichloromethyl) pyridine) for in vivo antitumor activity and also to discover related ring structures and sulfur analogues that might exhibit superior antitumor activity in vivo. Several new analogues of PEN and related structural variants have been synthesized and evaluated in vivo against MX-1 human breast tumor xenograft implanted subcutaneously (sc), although none of them demonstrated significant activity.

Metabolism of cyclophosphamide by purified cytochrome P-450 from microsomes of phenobarbital-treated rats

Incubation of [3H]-sidechain-labeled and [14C]-C(4)-ring-labeled cyclophosphamide (CPA) with purified cytochrome P-450 from liver microsomes of rats treated with phenobarbital resulted in the production of a major metabolite that contained both labels, was unaffected by diazomethane, possessed high polarity, was identical in TLC and HPLC behavior to a synthetic standard, didechlorodihydroxy -CPA, and was converted to CPA and bis(2-chloroethyl)amine by thionyl chloride . These results indicate that phenobarbital-inducible cytochrome P-450 is able to dechlorinate CPA and may account, in part, for the inability of phenobarbital to enhance the therapeutic activity and toxicity of this important anticancer and immunosuppressive agent.