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Detoxification of cyclophosphamide is effected, in part, by hepatic class 1 aldehyde dehydrogenase (ALDH-1)-catalyzed oxidation of aldophosphamide, a pivotal aldehyde intermediate, to the nontoxic metabolite, carboxyphosphamide. This enzyme is found in erythrocytes as well. Detoxification of aldophosphamide may also be effected by enzymes, viz. certain aldo-keto reductases, that catalyze the reduction of aldophosphamide to alcophosphamide. Such enzymes are also found in erythrocytes. Not known at the onset of this investigation was whether the contribution of erythrocyte ALDH-1 and/or aldo-keto reductases to the overall systemic detoxification of circulating aldophosphamide is significant. Thus, NAD-linked oxidation and NADPH-linked reduction of aldophosphamide catalyzed by relevant erythrocyte enzymes were quantified. ALDH-1-catalyzed oxidation of aldophosphamide (160 microM) to carboxyphosphamide occurred at a mean (+/- SD) rate of 5.0 +/- 1.4 atmol/min/rbc (red blood cell). Aldo-keto reductase-catalyzed reduction of aldophosphamide (160 microM) to alcophosphamide occurred at a much slower rate, viz. 0.3 +/- 0.2 atmol/min/rbc. Thus, at a pharmacologically relevant concentration of aldophosphamide, viz. 1 microM, estimated aggregate erythrocyte ALDH-1-catalyzed aldophosphamide oxidation, viz. 2.0 micromol/min, was only about 3% of estimated aggregate hepatic enzyme-catalyzed aldophosphamide oxidation, viz. 72 micromol/min; however, this rate is greater than the estimated flow-limited rate of aldophosphamide delivery to the liver by the blood, viz. 1.5 micromol/min. These observations/considerations suggest an important in vivo role for erythrocyte ALDH-1 in systemic aldophosphamide detoxification. Erythrocyte ALDH-1-effected oxidation of other aldehydes to their corresponding acids, e.g. retinaldehyde to retinoic acid, may also be of pharmacological and/or physiological significance since a wide variety of aldehydes are known to be substrates for ALDH-1.

Molecular determinants of cellular sensitivity to cyclophosphamide, long the mainstay of chemotherapeutic regimens used to treat metastatic breast cancer, include class 1 and class 3 aldehyde dehydrogenases (ALDH-1 and ALDH-3, respectively), which catalyze the detoxification of this agent. Thus, interindividual variation in the activity of either of these enzymes in breast cancers could contribute to the wide variation in clinical responses that are obtained when such regimens are used to treat these malignancies. Consistent with this notion, ALDH-1 levels in primary and metastatic breast malignancies were found to range from 1-276 and 8-160 mIU/g tissue, respectively, and those of ALDH-3 range from 1-242 and 6-97 mIU/g tissue, respectively. ALDH-1 and ALDH-3 levels in normal breast tissue predicted the levels of these enzymes in primary and metastatic breast malignancies present in the same individuals. Confirming and extending the observations of others, levels of glutathione, a molecular determinant of cellular sensitivity to various DNA cross-linking agents including cyclophosphamide, and of DT-diaphorase, glutathione S-transferases, and cytochrome P450 1A1, each of which is known to catalyze the detoxification/toxification of one or more anticancer agents (although not of cyclophosphamide), also varied widely in primary and metastatic breast malignancies. Given the wide range of ALDH-1, ALDH-3, and glutathione levels that were observed in malignant breast tissues, measurement of their levels in normal breast tissue and/or primary breast malignancies prior to the initiation of chemotherapy is likely to be of value in predicting the therapeutic potential, or lack thereof, of cyclophosphamide in the treatment of metastatic breast cancer, thus providing a rational basis for the design of individualized therapeutic regimens when treating this disease.

Cytosolic class-3 aldehyde dehydrogenase (ALDH-3) may help to protect organisms from certain environmental aldehydes by catalysing their detoxification. Consistent with this notion are the reports that relatively high levels of this enzyme are present in tissues, e.g. stomach mucosa and lung, that are so-called ports of entry for such agents. Further, it is found in human saliva. The present investigation revealed that small amounts of this enzyme are also present in human salivary glands; mean values for ALDH-3 activities (NADP-dependent enzyme-catalysed oxidation of benzaldehyde) in cytosolic fractions prepared from submandibular and parotid glands were 52 (range: 29-92) and 44 (range: 13-73) mIU/g tissue, respectively. Essentially identical or slightly lower levels of this enzyme activity were found in pleomorphic adenomas, an undifferentiated carcinoma, and an adenocystic carcinomas, of the parotid gland. On the other hand, Warthin tumours, and mucoepidermoid carcinomas of the parotid gland exhibited relatively elevated levels of ALDH-3 activity; mean values were 1200 (range: 780-1880) and 810 (range: 580-1200) mIU/g tissue, respectively. The ALDH-3 found in normal salivary glands was, as judged by physical, immunological and kinetic criteria, identical to human stomach mucosa ALDH-3 whereas the ALDH-3 present in Warthin tumours, and mucoepidermoid carcinomas, of the parotid gland appeared to be a subtle variant thereof. Qualitatively paralleling the relatively elevated ALDH-3 levels in mucoepidermoid carcinomas and Warthin tumours were relatively elevated levels of glutathione S-transferase (alpha and pi) and DT-diaphorase. As was the case with ALDH-3 levels, glutathione S-transferase (alpha and pi) and DT-diaphorase levels were not elevated in pleomorphic adenomas. Glutathione S-transferase mu was not detected in the two normal parotid gland samples, or in the single pleomorphic adenoma sample, tested. It was found in the single mucoepidermoid carcinoma sample, and in one of the two Warthin tumour samples tested. Cellular levels of ALDH-3, glutathione S-transferases and/or DT-diaphorase could be useful criteria when the decision to be made is whether a salivary gland tumour is a mucoepidermoid carcinoma. ALDH-3 and glutathione S-transferases are known to catalyse the detoxification of two agents that are used to treat salivary gland tumours, viz. cyclophosphamide and cisplatin, respectively. Thus, elevated levels of these enzymes in the mucoepidermoid carcinomas must account for, or at least contribute to, the relative ineffectiveness of these agents when used to treat this tumour.

Interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF alpha) protect normal human hematopoietic progenitors from the toxicity of 4-hydroperoxycyclophosphamide (4-HC). Aldehyde dehydrogenase Class 1 (ALDH-1) is the enzyme that inactivates 4-HC. Diethylaminobenzaldehyde (DEAB), a competitive inhibitor of ALDH-1, was shown to prevent the protective effects of IL-1 and TNF alpha. In this study, we examined the effect of IL-1 and TNF alpha on the expression of ALDH-1 in normal bone marrow as well as malignant cells. ALDH-1 mRNA and protein were quantified using Northern and Western blotting, respectively. In addition, the ALDH-1 enzyme activity in untreated as well as IL-1 and TNF alpha treated bone marrow cells was determined spectrophotometrically. The role of glutathione (GSH) in the protection against 4-HC toxicity was also studied. The results show that pretreatment with IL-1 and TNF alpha for 6 h or 20 h increase the expression of ALDH-1 mRNA and protein, respectively, in human bone marrow cells. In contrast, IL-1 and TNF alpha treatment did not affect the ALDH-1 expression in several leukemic and solid tumor cell lines, regardless of whether or not ALDH-1 is expressed constitutively. Furthermore, the ALDH-1 enzyme activity was significantly induced in bone marrow cells after 20 h pre-treatment with IL-1 and TNF alpha. Finally, the depletion of or inactivation of GSH did not affect the protection against 4-HC toxicity. In conclusion, inhibition of the protection from 4-HC toxicity by DEAB, together with the increase in ALDH-1 expression and activity, provide strong evidence that IL-1 and TNF alpha mediate their protective action, at least partially, through ALDH-1.

The cytosolic class aldehyde dehydrogenase (ALDH-3) present in human normal tissues/secretions is apparently much less able to catalyze the oxidation aldophosphamide to carboxyphosphamide than is the ALDH-3 present in human tumor cells/tissues, suggesting that the former may be less able to protect cells from the cytotoxic action of cyclophosphamide, mafosfamide, and other oxazaphosphorines. To test this notion, relatively large and approximately equal amounts of human normal stomach mucosa ALDH-3 and catechol-induced human breast adenocarcinoma MCF-7/0 ALDH-3 were first electroporated into cells (MCF-7/0) that constitutively express only very small amounts of the enzyme. The resultant preparations were then tested for sensitivity to mafosfamide. ALDH-3 activities (NADP-dependent catalysis of benzaldehyde oxidation) were 1.7, 212, and 183 mlU/10(7) cells in sham-electroporated MCF-7/0 cells, and MCF-7/0 cells electroporated with stomach mucosa ALDH-3 and catechol-induced MCF-7/0 ALDH-3, respectively. LC90 values (concentrations of mafosfamide required to effect a 90% cell kill) were 62, 417, and >1,000 microM, respectively. The three preparations were equisensitive to phosphoramide mustard (LC90 = approximately 850 microM). Inclusion of benzaldehyde in the drug exposure medium fully restored the sensitivity of MCF-7/0 cells electroporated with either enzyme to mafosfamide. These observations support the notions that 1) cellular sensitivity to the oxazaphosphorines decreases as the cellular content of ALDH-3 increases, 2) the foregoing is the consequence of ALDH-3-catalyzed oxidation (thus detoxification) of aldophosphamide, and 3) the ALDH-3 present in at least some tumor cells/tissues is a slight variant of the ALDH-3 present in normal tissues/secretions. Furthermore, they illustrate the utility of electroporation used as a tool to determine whether a given enzyme, or even more generally, protein or other macromolecule, is a determinant of cellular sensitivity to a given cytotoxic agent.

Human saliva was tested for the presence of cytosolic class 3 aldehyde dehydrogenase, glutathione S-transferases alpha, mu, and pi, and DT-diaphorase, enzymes that are known to catalyze the biotransformation of many xenobiotics, including some that are carcinogens and some that are antineoplastic agents. Each of these enzymes was found to be present in this fluid. Inducers of these enzymes are known to be abundantly present in the human diet, especially in certain vegetables and fruits. Further investigation revealed that the salivary content of these enzymes rapidly, coordinately, and markedly increased upon daily consumption of relatively large amounts of coffee or broccoli. The enzyme activities of interest rapidly returned to basal levels when these substances were removed from the diet. Given the important role that cytosolic class 3 aldehyde dehydrogenase, the glutathione S-transferases, and DT-diaphorase are thought to play in determining the carcinogenic potential of some cancer-producing agents as well as the cytotoxic potential of some antineoplastic agents, and assuming that their salivary levels reflect their tissue levels, quantification of the salivary content of one or more of these enzymes, a noninvasive and relatively easy undertaking, could be useful in: (a) preliminarily assessing the chemopreventive potential of various diets and drugs; (b) establishing the optimal dose and schedule in Phase I clinical trials for any putatively chemopreventive diets or drugs of interest; and (c) the rational selection and use of chemotherapeutic agents, since several are inactivated, and a few are activated, by these enzymes; alternatively, the antineoplastic agent could be selected first and then a diet that enables the agent to achieve its full therapeutic potential would be selected based on whether high or low enzyme activity would be favorable in that regard. Such measurements may also be useful as an indicator when exposure to carcinogenic/teratogenic/otherwise toxic environmental/industrial/dietary agents that induce these enzymes is suspected.

Primary cyclophosphamide-naive clonogenic blasts from 32 patients with newly diagnosed acute lymphoblastic leukemia (ALL) were tested for their in vitro sensitivity to an "activated" cyclophosphamide, viz., mafosfamide, using leukemic progenitor cell (LPC) colony assays. Marked interpatient variation in the responses of LPC from newly diagnosed patients to mafosfamide prompted assessment of mafosfamide sensitivity in relation to more frequently measured parameters of newly diagnosed ALL. Only immunophenotype and sex showed a significant association with the intrinsic mafosfamide sensitivity of LPC. LPC from T-lineage ALL patients were more resistant to mafosfamide than LPC from B-lineage ALL patients, as reflected by 1.8-fold and 4.3-fold higher mean SF10 and SF20 (surviving fractions of ALL LPC of 10 and 20 microM mafosfamide, respectively) values. LPC from male patients were more resistant to mafosfamide than LPC from female patients, as reflected by 1.9-fold and 4.8-fold higher mean SF10 and SF20 values. In comparison to T-lineage ALL patients, a significantly greater fraction of B-lineage ALL patients had mafosfamide-sensitive LPC with SF10 values of < 0.25 (61% vs 11%, P = 0.01). Notably, all four cases exhibiting resistance to mafosfamide, i.e., SF20 > or = 0.5, were males with T-lineage ALL. In order to exclude the influence of sex as a confounding factor in the observed immunophenotype-mafosfamide sensitivity association, we also compared the mafosfamide sensitivities of LPC from male patients only. The means of SF10, and SF20 values of LPC from male T-lineage ALL patients were 1.5- and 3.2-fold higher than those of LPC from male B-lineage ALL patients (P < 0.1). Thus, in the male patient subgroup, the immunophenotype-mafosfamide sensitivity association remained significant.

The class-3 aldehyde dehydrogenase that is overexpressed (> 100-fold) in human breast adenocarcinoma MCF-7/0 cells made resistant (> 30-fold as judged by LC90s) to oxazaphosphorines, such as mafosfamide, by growing them in the presence of polycyclic aromatic hydrocarbons, e.g., methylcholanthrene (3 microM for 5 days), was isolated and characterized. Its physical and catalytic properties were identical to those of the prototypical human stomach mucosa cytosolic class-3 aldehyde dehydrogenase, type-1 ALDH-3, except that it catalyzed, though not very rapidly, the oxidation of aldophosphamide, whereas the stomach mucosa enzyme essentially did not; hence, it was judged to be a slight variant of the prototypical enzyme. Carcinogens that are not ligands for the Ah receptor, barbiturates known to induce hepatic cytochrome P450s, steroid hormones, an antiestrogen, and oxazaphosphorines did not induce the enzyme or the largely oxazaphosphorine-specific acquired resistance. Whereas methylcholanthrene induced (a) resistance to mafosfamide and (b) class-3 aldehyde dehydrogenase activity, as well as glutathione S-transferase and DT-diaphorase activities, in the estrogen receptor-positive MCF-7/0 cells, it did not do so in two other human breast adenocarcinoma cell lines, MDA-MB-231 and SK-BR-3, each of which is estrogen receptor negative. Expression of the class-3 aldehyde dehydrogenase and the loss of sensitivity to mafosfamide by polycyclic aromatic hydrocarbon-treated MCF-7/0 cells were transient; each returned to essentially basal levels within 15 days when the polycyclic aromatic hydrocarbon was removed from the culture medium. Insensitivity to the oxazaphosphorines on the part of polycyclic aromatic hydrocarbon-treated MCF-7/0 cells was not observed when exposure to mafosfamide (30 min) was in the presence of benzaldehyde or octanal, each a relatively good substrate for cytosolic class-3 aldehyde dehydrogenases, whereas it was retained when exposure to mafosfamide was in the presence of acetaldehyde, a relatively poor substrate for these enzymes. These observations demonstrate that ligands for the Ah receptor can induce a transient, largely oxazaphosphorine-specific, acquired cellular resistance, and they are consistent with the notion that elevated levels of a cytosolic class-3 aldehyde dehydrogenase nearly identical to the prototypical type-1 class-3 aldehyde dehydrogenase expressed by human stomach mucosa account for the Ah receptor ligand-induced oxazaphosphorine-specific acquired resistance, most probably by catalyzing the detoxification of aldophosphamide.

One of the metabolites found in the urine of mammals given the prodrug cyclophosphamide is alcophosphamide, an alcohol. It is most probably generated from cyclophosphamide via aldophosphamide, an aldehyde which otherwise can directly give rise to phosphoramide mustard; the latter effects the cytotoxic action of cyclophosphamide and other oxazaphosphorines. It has already been demonstrated that horse liver alcohol dehydrogenase catalyzes the reduction of aldophosphamide to alcophosphamide. Herein, we report that aldose reductase and aldehyde reductase purified from human placenta also catalyze this reaction. The Km values for aldose reductase- and aldehyde reductase-catalyzed reduction of aldophosphamide to alcophosphamide were 0.15 and 1.6 mM, respectively. Aldose reductase and aldehyde reductase accounted for 94 and 6%, respectively, of total placental pyridine nucleotide-dependent enzyme-catalyzed aldophosphamide (160 microM) reduction. Aldose reductase-catalyzed reduction of aldophosphamide appeared to be noncompetitively inhibited by sorbinil; the Ki value was 0.4 microM. The in vivo significance of these observations is uncertain but could be of some magnitude since alcophosphamide is known to be only weakly cytotoxic.

Biotransformation of the biologically and pharmacologically important aldehydes, retinaldehyde and aldophosphamide, is mediated, in part, by NAD(P)-dependent aldehyde dehydrogenases catalyze the oxidation of the aldehydes to their respective acids, retinoic acid and carboxyphosphamide. Not known at the onset of this investigation was which of the several known human aldehyde dehydrogenases (ALDHs) catalyze these reactions. Thus, human liver aldehyde dehydrogenases were chromatographically resolved and the ability of each to catalyze the oxidation of retinaldehyde and aldophosphamide was assessed. Only one, namely ALDH-1, catalyzed the oxidation of retinaldehyde; the Km value was 0.3 microM. Three, namely ALDH-1, ALDH-2 and succinic semialdehyde dehydrogenase, catalyzed the oxidation of aldophosphamide; Km values were 52, 1193, and 560 microM, respectively. ALDH-4, ALDH-5 and betaine aldehyde dehydrogenase did not catalyze the oxidation of either aldophosphamide or retinaldehyde. ALDH-1 and succinic semialdehyde dehydrogenase accounted for 64 and 30%, respectively, of the total hepatic aldehyde dehydrogenase-catalyzed aldophosphamide (160 microM) oxidation. ALDH-1-catalyzed oxidation of aldophosphamide was noncompetitively inhibited by chloral hydrate; the Ki value was 13 microM. ALDH-2- and succinic semialdehyde dehydrogenase-catalyzed oxidation of aldophosphamide was relatively insensitive to inhibition by chloral hydrate. These observations strongly suggest an important in vivo role for ALDH-1 in the catalysis of retinaldehyde and aldophosphamide biotransformation. Succinic semialdehyde dehydrogenase-catalyzed biotransformation of aldophosphamide may also be of some in vivo importance.

NAD(P)-linked aldehyde dehydrogenases catalyze the oxidation of a wide variety of aldehydes. Thirteen of these enzymes have been identified in mouse tissues; eleven are found in the liver. Some are substrate-nonspecific; others are relatively substrate-specific. The present investigation sought to determine which of these enzymes are operative in catalyzing the oxidation of retinaldehyde to retinoic acid, a metabolite of vitamin A that promotes the differentiation of epithelial and other cells. Spectrophotometric and HPLC assays were used for this purpose. Enzyme-catalyzed oxidation of retinaldehyde (25 microM) was restricted to the cytosol (105,000 g supernatant fraction) and occurred at a rate of 211 nmol/min/g liver; oxidation of acetaldehyde (4 mM) by this fraction proceeds about ten times faster. At least 90% of this activity was NAD dependent. Of the approximately 10% that was apparently NAD independent, two-thirds was inhibited by 1 mM pyridoxal, a known inhibitor of aldehyde oxidase. Of the six cytosolic aldehyde dehydrogenases, only two, viz. AHD-2 and AHD-7, catalyzed the oxidation of retinaldehyde to retinoic acid. An additional NAD-dependent enzyme, viz. xanthine oxidase (dehydrogenase form), also catalyzed the reaction. Catalysis by AHD-2 accounted for more than 90% of the total NAD-dependent activity. Km values were 0.7, 0.6 and 0.9 microM, respectively, for the AHD-2-, AHD-7- and xanthine oxidase (dehydrogenase form)-catalyzed reaction. AHD-4, an aldehyde dehydrogenase found in the cytosol of mouse stomach epithelium and cornea, did not catalyze the reaction.

Several mouse aldehyde dehydrogenases catalyze the detoxification of aldophosphamide, the pivotal metabolite of the prodrugs cyclophosphamide, mafosfamide, and other oxazaphosphorines. N-Isopropyl-p-formylbenzamide, a major metabolite of procarbazine, was found to be an excellent substrate (Km = 0.84 microM) for at least one of these enzymes, namely, mouse aldehyde dehydrogenase-2. The Km for mouse aldehyde dehydrogenase-2-catalyzed detoxification of aldophosphamide is 16 microM. Thus, competition between N-isopropyl-p-formylbenzamide and aldophosphamide for the catalytic site on the enzyme should strongly favor the former, and the rate at which aldophosphamide is detoxified should be markedly retarded. Mouse L1210/OAP and P388/CLA leukemia cells are relatively insensitive to the oxazaphosphorines because they contain large amounts of mouse aldehyde dehydrogenase-2. As predicted, N-isopropyl-p-formylbenzamide markedly potentiated the cytotoxic action of mafosfamide against these cells. Mouse L1210/0 and P388/0 lack the enzyme. Again as expected, N-isopropyl-p-formylbenzamide essentially did not potentiate the cytotoxic action of mafosfamide against these cells. Certain mouse and human hematopoietic progenitor cells also contain an aldehyde dehydrogenase that catalyzes the detoxification of aldophosphamide, but the specific identity of this enzyme remains to be established. N-Isopropyl-p-formylbenzamide potentiated the cytotoxic action of mafosfamide against these cells as well. Clinically, procarbazine and the oxazaphosphorines are used to treat certain neoplastic diseases. Frequently, they are used in combination. Our findings demonstrate the potential for both desirable and undesirable drug interactions when these agents are used concurrently. Similar drug interactions can be expected when other substrates for, or inhibitors of, the relevant aldehyde dehydrogenases, e.g., chloramphenicol, chloral hydrate, and methyltetrazolethiol-containing cephalosporins, are co-administered with the oxazaphosphorines.

Aldophosphamide, the penultimate cytotoxic metabolite of cyclophosphamide, can be detoxified by an oxidation reaction catalyzed by certain aldehyde dehydrogenases. The selective toxicity of cyclophosphamide is due, at least in part, to a greater expression of the relevant aldehyde dehydrogenase activity in normal cells relative to that expressed in certain tumor cells. Not known at the onset of this investigation was which of the several known mouse aldehyde dehydrogenases catalyze this reaction. Twelve enzymes that catalyze the NAD(P)-linked oxidation of aldophosphamide, acetaldehyde, benzaldehyde, and/or octanal were chromatographically resolved from mouse liver. Four of these appear to be novel; four others were determined to be betaine aldehyde dehydrogenase, succinic semialdehyde dehydrogenase, glutamic gamma-semialdehyde dehydrogenase, and xanthine oxidase (dehydrogenase). An additional aldehyde dehydrogenase, namely AHD-4, was semipurified from stomach. The stomach enzyme and nine of the hepatic enzymes catalyze the oxidation of aldophosphamide. Km values for these reactions range from 16 microM to 2.5 mM. The relevant aldehyde dehydrogenase of major importance varies with the tissue. In the liver, the major cytosolic aldehyde dehydrogenase, namely AHD-2, accounts for greater than 60% of total hepatic aldehyde dehydrogenase-catalyzed aldophosphamide (160 microM) detoxification. Succinic semialdehyde dehydrogenase (AHD-12) and three of the novel hepatic aldehyde dehydrogenases, namely AHD-8, AHD-10, and AHD-13, also contribute significantly to total hepatic aldehyde dehydrogenase-catalyzed aldophosphamide detoxification. In the stomach, AHD-4 and AHD-8 account for approximately 86% of total aldehyde dehydrogenase-catalyzed aldophosphamide (160 microM) detoxification. AHD-2 was not found in this tissue. Of all the aldehyde dehydrogenases examined, AHD-2 and AHD-8 were estimated to be the most efficient catalysts of aldophosphamide oxidation. Thus, these enzymes would seem most likely to be operative when tumor cells acquire aldehyde dehydrogenase-mediated cyclophosphamide resistance.

The ex vivo sensitivity of murine multipotent (CFU-GEMM) and committed (CFU-Mk, CFU-GM, BFU-E and CFU-E) hematopoietic progenitor cells to mafosfamide was quantified with and without concurrent exposure to cyanamide, an inhibitor of aldehyde dehydrogenase activity. In the absence of cyanamide, CFU-GEMM, CFU-Mk and CFU-GM were approximately equisensitive to mafosfamide while the erythroid progenitors were more sensitive to the drug. Cyanamide potentiated the cytotoxicity of mafosfamide toward CFU-GEMM and CFU-Mk, but not toward CFU-GM, BFU-E and CFU-E. Cellular aldehyde dehydrogenases are known to catalyze the oxidation of 4-hydroxycyclophosphamide/aldophosphamide, the major intermediate in cyclophosphamide and mafosfamide activation, to the relatively nontoxic acid, carboxyphosphamide. Thus, our findings indicate that 1) murine CFU-GEMM contain the relevant aldehyde dehydrogenase activity, and 2) the relevant aldehyde dehydrogenase activity is retained upon differentiation to progenitors committed to the megakaryocytoid lineage, but lost upon differentiation to progenitors committed to the granulocytoid/monocytoid and erythroid lineages. The relative insensitivity of CFU-GM to mafosfamide is apparently due to a cellular determinant that influences their sensitivity to all cross-linking agents since CFU-GM were found to be relatively insensitive to non-oxazaphosphorine cross-linking agents as well.

The sensitivity of cultured L1210 and P388 cells sensitive (L1210/0, P388/0) and resistant (L1210/OAP, P388/CLA) to oxazaphosphorines, to 4-hydroperoxycyclophosphamide, ASTA Z-7557, phosphoramide mustard, and acrolein was determined in the absence and presence of known (disulfiram, diethyldithiocarbamate, cyanamide) or suspected [ethylphenyl(2-formylethyl)phosphinate] inhibitors of aldehyde dehydrogenase activity. The L1210/OAP cell line is resistant specifically to the oxazaphosphorines; P388/CLA cells are partially cross-resistant to other cross-linking agents. All four inhibitors of aldehyde dehydrogenase activity potentiated the cytotoxic action of the oxazaphosphorines, 4-hydroperoxycyclophosphamide and ASTA Z-7557, against L1210/OAP and P388/CLA cells; in the presence of a sufficient amount of inhibitor, sensitivity was essentially fully restored in both cases. The inhibitors did not potentiate the cytotoxic action of the nonoxazaphosphorines, phosphoramide mustard and acrolein, against these cell lines. The cytotoxic action of the oxazaphosphorines and nonoxazaphosphorines against L1210/0 and P388/0 cells was not potentiated by any of the aldehyde dehydrogenase inhibitors. Inhibitors of xanthine oxidase or aldehyde oxidase activities did not potentiate the cytotoxic action of the oxazaphosphorines against L1210/OAP cells. These observations strongly suggest that (a) aldehyde dehydrogenase activity is an important determinant with regard to the sensitivity of a cell population to the oxazaphosphorines; (b) L1210/0 and P388/0 cells lack the relevant aldehyde dehydrogenase activity; (c) the phenotypic basis for the resistance to oxazaphosphorines by L1210/OAP cells is aldehyde dehydrogenase activity; and (d) the major reason that P388/CLA cells are resistant to oxazaphosphorines is aldehyde dehydrogenase activity.

Plasma half-life and area under the curve (AUC) values for cyclophosphamide were determined in patients given this agent iv at doses of 50-60 mg/kg/infusion. Apparent plasma half-life and AUC values for the metabolites 4-hydroxycyclophosphamide and phosphoramide mustard were also determined in some of these patients. Disappearance from the plasma of the parent compound as well as that of the metabolites was approximately first-order. Plasma half-life values for cyclophosphamide ranged from 45 to 480 mins; AUC values ranged from 10 to 188 mM X min. As expected, AUC values for cyclophosphamide increased approximately linearly with an increase in its plasma half-life. Apparent plasma half-life values for 4-hydroxycyclophosphamide and phosphoramide mustard increased approximately linearly with an increase in plasma half-life values for cyclophosphamide; the slopes of these relationships were 1.35 and 1.97, respectively, but did not quite extrapolate to zero. AUC values for 4-hydroxycyclophosphamide and phosphoramide mustard remained approximately constant at about 5 and 15 mM X min, respectively, over the relatively wide range of plasma half-life and AUC values obtained for cyclophosphamide. On the basis of these observations we suggest that (a) changes in the rate of cyclophosphamide hydroxylation, effected by whatever means, will not alter the systemic therapeutic and toxic responses to a given dose of cyclophosphamide, given that the cytotoxic effects of this agent are directly proportional to AUC values of 4-hydroxycyclophosphamide and/or phosphoramide mustard, and (b) in most cases, 4-hydroxycyclophosphamide, and not phosphoramide mustard, is likely to be the circulating metabolite of therapeutic importance in humans since the AUC values for phosphoramide mustard exceeded those for 4-hydroxycyclophosphamide by only a factor of 3 and tumor and bone marrow cells proliferating in culture are generally substantially (8-25-fold) more sensitive to 4-hydroxycyclophosphamide.