Effect of the aldehyde dehydrogenase inhibitor, cyanamide, on the ex vivo sensitivity of murine multipotent and committed hematopoietic progenitor cells to mafosfamide (ASTA Z 7557)

Sensitivity of aldehyde dehydrogenases in murine tumor and hematopoietic progenitor cells to inhibition by chloral hydrate as determined by the ability of chloral hydrate to potentiate the cytotoxic action of mafosfamide

Several murine aldehyde dehydrogenases, most notably AHD-2, are known to catalyze the detoxification of cyclophosphamide, mafosfamide, and other oxazaphosphorines. Thus, cellular sensitivity to these agents decreases as the relevant aldehyde dehydrogenase activity increases, and vice versa. Chloral hydrate is a sedative/hypnotic agent that is sometimes administered to patients being treated with cyclophosphamide. It is known to inhibit some, but not all, aldehyde dehydrogenases. Murine (CFU-S, CFU-GEMM and CFU-Mk) and human (CFU-Mix, CFU-GM, BFU-E and CFU-Mk) hematopoietic progenitor cells, as well as murine oxazaphosphorine-resistant (L1210/OAP and P388/CLA) tumor cells, are known to contain the relevant aldehyde dehydrogenase activity but the identity of the specific enzyme present in the normal cells is unknown and may be different than that, namely AHD-2, present in neoplastic cells. In that event, the potential exists to inhibit the detoxification of the oxazaphosphorines in tumor cells without inhibiting this event in normal cells; the net effect of such a selective inhibition would be to increase the margin of safety of the oxazaphosphorines. In ex vivo experiments, chloral hydrate markedly potentiated the antitumor activity of mafosfamide against oxazaphosphorine-resistant L1210/OAP and P388/CLA cells. It did not potentiate the cytotoxic action of mafosfamide against any of the murine or human hematopoietic cells tested, even at concentrations which fully restored the sensitivity of the resistant tumor cell lines to this agent. One explanation for these observations is that hematopoietic progenitor, and the resistant tumor, cells express different relevant aldehyde dehydrogenases and that these aldehyde dehydrogenases differ in their sensitivity to inhibition by chloral hydrate. Consistent with this notion were the observations that AHD-2 was exquisitely sensitive to inhibition by chloral hydrate, whereas two other aldehyde dehydrogenases that also catalyze the detoxification of aldophosphamide, namely AHD-12a, b and AHD-13, were relatively unaffected.

Identification of human liver aldehyde dehydrogenases that catalyze the oxidation of aldophosphamide and retinaldehyde

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.

Biotransformation of mafosfamide in P388 mice leukemia cells: intracellular 31P-NMR studies

The intracellular transformation of cis-mafosfamide has been studied in P388 mice leukemia cells using 31P-NMR spectroscopy. For this purpose the cells were entrapped in low-gelling-temperature agarose threads. Internal pH of the cells, determined from the position of the intracellular inorganic phosphate, was 7.2. The cell membrane was permeable to 4-hydroxycyclophosphamide and aldophosphamide and less permeable to phosphoramide mustard. 4-Ketocyclophosphamide and carboxyphosphamide signals were not detectable in cells either sensitive or resistant to oxazaphosphorine treatment.

Bone marrow purging with mafosfamide--a critical survey

Autologous bone marrow transplantation (ABMT) is increasingly used to consolidate remissions, primarily in hematological disease. Various purging strategies have been developed to minimize the risk of reimplantation of tumor cells with the bone marrow autotransplant. Pharmacological purging with the oxazaphosphorine derivative mafosfamide has been studied extensively, and recent clinical data suggest that purging with mafosfamide may translate into superior remission duration if compared to nonpurged ABMT in acute leukemia. Chemical and experimental data relevant to mafosfamide-purging and clinical results are reviewed, with special emphasis on safety aspects.