Effects of L-histidine and its structural analogues on human N-myristoyltransferase activity and importance of EEVEH amino acid sequence for enzyme activity.

Myristoyl-CoA:protein N-myristoyltransferase (NMT) is an essential eukaryotic enzyme that catalyzes the cotranslational transfer of myristate to the NH2-terminal glycine residue of a number of important proteins of diverse function. Human NMT (hNMT) activity was found to be activated by L-histidine in a concentration-dependent manner. In contrast, two structural analogues of L-histidine, L-histidinol and histamine, inhibited hNMT activity in a noncompetitive manner with half-maximal inhibitions of 18 and 1.5 mM, respectively. The inhibition of hNMT activity by L-histidinol was reversed by a 2-fold molar excess of L-histidine, suggesting that L-histidine and L-histidinol were competing for a common site on NMT. Kinetic data indicated that whereas L-histidine enhanced the Vmax, both L-histidinol and histamine decreased the Vmax; none of these compounds altered the Km. Our studies suggest that L-histidine and its analogues may be interacting with His-293, involved in myristoyl-CoA transfer, rather than His-218, and implicated in the transfer of myristoyl-CoA to the peptide substrates. Site-directed mutagenesis of His-293, Val-291, and Glu-290 resulted in proteins with no measurable NMT activity. The most conserved region in the catalytic domain EEVEH (289-293) is critical for the myristoyl-CoA transfer in the NMT-catalyzed reactions. This region will be useful for the design of regulators of NMT function.

R-deprenyl and R-2-heptyl-N-methylpropargylamine prevent apoptosis in cerebellar granule neurons induced by cytosine arabinoside but not low extracellular potassium.

R-Deprenyl and R-2-heptyl-N-methylpropargylamine (R-2-HMP) are compounds that have been shown to reduce neuronal death in various in vitro and in vivo models involving apoptosis but do not always prevent apoptosis. In the present study we have examined the effects of these compounds and their S enantiomers on cytosine arabinoside (ara C)-induced apoptosis and low K+-induced apoptosis in cerebellar granule cells in primary culture. It was found that R-deprenyl and R-2-HMP could prevent ara C-induced apoptosis with an EC50 around 10(-9) M but could not prevent low K+-induced apoptosis. S-Deprenyl and S-2-HMP did not prevent apoptosis under any conditions but were found to antagonize the antiapoptotic actions of R-deprenyl and R-2-HMP. Using the fluorescent mitochondrial dye chloromethyltetramethylrhodamine methyl ester it was found that there was a loss of mitochondrial function in cerebellar granule cells exposed to ara C but not low K + medium. R-Deprenyl and R-2-HMP prevented the ara C-induced loss of mitochondrial function. It is concluded that R-deprenyl and R-2-HMP prevent apoptosis of cerebellar granule cells by a mechanism that is independent of monoamine oxidase inhibition and that they act on the same site to prevent specifically apoptosis involving a loss of mitochondrial membrane potential, possibly p53-dependent apoptosis.

L-histidinol selectively modulates daunomycin toxicity in normal and tumorigenic kidney epithelial cells.

L-Histidinol protects normal cells from anticancer drugs while enhancing the ability of these agents to eradicate tumor cells. We now report that this attribute, previously documented in normal and tumor cells of fibroblastic or myeloid origin, extends to epithelial lines. Clonogenic cell survival assays showed that L-histidinol protected the normal Madin-Darby canine kidney (MDCK) epithelial cell line from daunomycin (DAU) toxicity, but enhanced DAU toxicity in MDCK-T1, a tumorigenic derivative of MDCK. The protection of MDCK cells from DAU by L-histidinol was improved by increasing L-histidine in the media, a condition known to diminish L-histidinol's capacity to inhibit protein synthesis. In contrast, similar conditions markedly diminished the capacity of L-histidinol to enhance DAU killing of MDCK-T1 cells. These data suggest that the differential effects of L-histidinol on DAU toxicity cannot be attributed totally to its ability to inhibit protein synthesis.

Mimetics of L-histidinol which selectively modulate daunomycin toxicity in normal and tumorigenic epithelial cells.

In the accompanying publication, it was shown that L-histidinol protected the normal Madin-Darby canine kidney (MDCK) epithelial cell line from daunomycin (DAU) toxicity, but enhanced DAU toxicity in MDCK-T1, a tumorigenic derivative of MDCK. The protection of MDCK cells from DAU by L-histidinol was also shown to be independent of its ability to inhibit protein synthesis. Here, clonogenic cell survival assays show that imidazole was as effective as L-histidinol in protecting MDCK cells from DAU, but had less impact on MDCK-T1 line. Certain antieicosanoids and antihistamines mimicked, to varying extents, the DAU-modulating action of L-histidinol. These data suggest that the selective modulation of DAU toxicity by L-histidinol involves both inhibition of protein synthesis and unknown action(s) attributable to its imidazole moiety and that other pharmacological agents are modulators of anticancer drug toxicity.

L-histidinol reverses resistance to cisplatinum and other antineoplastics in a tumorigenic epithelial cell line.

L-Histidinol protects normal cells from various anticancer drugs, while enhancing the toxicity of the same agents in drug-sensitive and multidrug-resistant tumor cells. Here, we report that L-histidinol circumvents a novel form of multiple-drug resistance in the MDCK-T1 line, a tumorigenic derivative of the phenotypically-normal Madin-Darby canine kidney (MDCK) epithelial cell line. Clonogenic cells survival assays showed that, compared to the parental MDCK line, the MDCK-T1 line was resistant to 1,3-bis(2-chloroethyl)-1-nitrosourea (approximately 15 fold), to cisplatinum (approximately 10 fold), to 5-fluorouracil (approximately 10-fold) and to cytosine arabinoside (approximately 15-fold). L-Histidinol reversed the resistance of MDCK-T1 cells to these anticancer drugs while protecting the parental MDCK line from these agents. These studies indicate that L-histidinol reverses a unique form of drug-resistance in MDCK-T1 cells by a mechanism dependent upon protein synthesis inhibition.

Results of a clinical trial in humans with refractory cancer of the intracellular histamine antagonist, N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine-HCl, in combination with various single antineoplastic agents.

PURPOSE: We assessed N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine-HCl (DPPE) potentiation of chemotherapy in vitro and performed a pharmacokinetic study and phase I/II trial of DPPE, combined with various single agents, in patients with advanced refractory cancer. PATIENTS AND METHODS: In vitro chemopotentiation by DPPE was assessed in drug-sensitive and -resistant (multidrug resistant-positive [MDR+]) human tumor cells using a colony survival assay. The effect of DPPE and verapamil on the intracellular concentration of daunorubicin in MDR+ cells was compared. For the clinical study, subjects with progressive malignancy received a weekly infusion of a maximally tolerated dose of DPPE (240 mg/m2) over 80 or 440 minutes, in conjunction with a single chemotherapy drug to which, in most cases, the patient's tumor was previously resistant. Concentrations of DPPE in blood and urine were determined by high-performance liquid chromatography (HPLC). RESULTS: In vitro, micromolar concentrations of DPPE potentiated (fivefold to 10-fold) chemotherapy cytotoxicity to both drug-sensitive and -resistant cells, but did not inhibit the p-glycoprotein pump; in vivo, serum levels of DPPE were 3 to 5 mumol/L at the end of 80 minutes and 1 to 2 mumol/L after 440 minutes of infusion. Of 48 patients monitored for a minimum of four DPPE/chemotherapy treatment cycles, 16 (33%) progressed, 12 (25%) stabilized, 12 (25%) improved, and eight (17%) responded (one complete and seven partial remissions). Four of 11 subjects who did not respond to the 80-minute infusion regimen improved with the 440-minute infusion; one had a partial remission of melanoma. In more than 600 patient-treatments, bone marrow toxicity was negligible (mean absolute neutrophil count [ANC] > 2.0 x 10(9)/L). Acute CNS symptoms associated with DPPE infusions were of relatively short duration (1 to 4 hours); delayed toxicity attributable to DPPE consisted of mild nausea and/or fatigue (1 to 2 days). CONCLUSION: Although preliminary, the results suggest that more structured trials should be performed to determine whether DPPE may increase the therapeutic index of certain chemotherapy drugs.

Enhanced cancer growth in mice administered daily human-equivalent doses of some H1-antihistamines: predictive in vitro correlates.

BACKGROUND: Present studies of drug-induced tumor growth promotion have evolved from earlier investigations into the mechanism of action of N,N-diethyl-2-[4-(phenylmethyl)phenoxy[ethanamine.HCl, a tamoxifen derivative which potently inhibits lymphocyte mitogenesis in vitro and stimulates tumor growth in vivo. It is thought that potency to bind to intracellular histamine receptors (HIC), some of which are on cytochromes P450, may correlate with tumor growth-promoting activity. PURPOSE: We assessed the effectiveness of five in vitro assays in predicting in vivo tumor growth stimulation by the H1-antihistamines loratadine, astemizole, cetirizine, hydroxyzine, and doxylamine. METHODS: Potency of each agent was ranked 1-5 in each of the following in vitro assays: 1) inhibition of [3H]histamine binding to microsomal HIC, 2) inhibition of histamine binding to microsomal P450, 3) inhibition of the P450-catalyzed demethylation of aminopyrine, 4) inhibition of lymphocyte mitogenesis, and 5) stimulation of tumor colony formation. An overall rank score was assigned to each drug and correlated with tumor growth stimulation in vivo. Two laboratories conducted in vivo studies in a blinded fashion. Female C57BL and C3H mice were given a subcutaneous injection on day 1 of syngeneic B16F10 melanoma cells (5 x 10(5)) or C-3 fibrosarcoma cells (1 x 10(5)), respectively. Mice were randomly assigned to treatment groups, then received a single, daily intraperitoneal injection of an estimated human-equivalent dose (or range of doses) of antihistamine or vehicle control for 18-21 days before being killed. Tumors were surgically removed and wet weights compared statistically among groups. RESULTS: The cumulative potency of each drug in affecting tumor growth or growth mechanisms in the five in vitro assays ranked as follows: Loratidine and astemizole ranked highest and were equally potent, followed in decreasing order by hydroxyzine, doxylamine, and cetirizine. A significant correlation (r = .97; P < .02) was observed between the rank order of potency of the antihistamines in all five in vitro assays and the rank order to enhance tumor growth in vivo: Loratidine and astemizole significantly (P < .001) promoted the growth of both melanoma and fibrosarcoma, hydroxyzine significantly (P < .001) promoted the growth of melanoma, while doxylamine and cetirizine did not promote the growth of either tumor. CONCLUSION: Data demonstrate that the in vitro assays predicted the propensity of each H1-antihistamine to stimulate cancer growth in vivo. IMPLICATION: These in vitro tests may prove valuable to screen potential tumor growth promoters.

Susceptibility of human colon carcinoma cells to anticancer drugs is enhanced by L-histidinol.

The effects of L-histidinol on cell cycle transit and anticancer drug susceptibility of four cultured human colon carcinoma cell lines have been examined. The lines studied included two lines, SW480 and SW 620, which were derived from the same patient as a grade III adenocarcinoma of the colon and a lymphatic metastatic form of colon carcinoma, respectively, as well as the human colon adenocarcinoma lines HT-29 and WiDr. L-Histidinol accentuates the vulnerability of all of these human carcinoma lines to several different classes of anticancer drugs, including 5-fluorouracil, the most effective and most commonly used single agent for metastatic colon carcinoma. The ability of L-histidinol to accentuate the sensitivity of these colocarcinoma lines to anticancer drugs, including those which are cell cycle-dependent, is in apparent contradiction to the finding that L-histidinol markedly slows cycle transit in all lines tested.

L-histidinol increases the vulnerability of cultured human leukemia and lymphoma cells to anticancer drugs.

L-Histidinol, a structural analogue of the essential amino acid L-histidine, enhances the toxicity of a variety of anticancer drugs for many tumor cells of animal origin. In this study, the effects of L-histidinol on the proliferation and anticancer drug susceptibility of two human tumor cell lines of lymphoid origin, Daudi and MOLT 4, have been examined. L-Histidinol increased the inherent capacity of six different antineoplastic agents to kill these human tumor cells, in a dose- and time-dependent manner, in spite of the observation that it slowed cell cycle progression in both lines.

Stimulation of malignant growth in rodents by antidepressant drugs at clinically relevant doses.

Tricyclic antidepressants, such as amitriptyline (Elavil), and the nontricyclic agent, fluoxetine (Prozac), bind to growth-regulatory intracellular histamine receptors, associated with anti-estrogen binding sites in microsomes and nuclei. The prototype anti-estrogen binding site/intracellular histamine receptor ligand, N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine HCl, inhibits normal cell proliferation in vitro but stimulates tumor growth in vivo. Because of their structural similarity to N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine HCl, we carried out studies to determine whether amitriptyline and fluoxetine stimulate tumor growth and/or development in rodents at concentrations relevant to the treatment of human depression (equivalent human dose range, approximately 100-150 mg/day for amitriptyline and approximately 20-80 mg/day for fluoxetine). All experiments were performed blinded. In studies of growth stimulation of transplantable syngeneic tumors, groups of mice were inoculated s.c. with C-3 fibrosarcoma cells or given i.v. or s.c. injections of B16f10 melanoma cells, followed 24 h later by daily i.p. injections of saline, amitriptyline, or fluoxetine. Tumor latency (fibrosarcoma), aggregate tumor weight (s.c. injected melanoma), or time to death from pulmonary metastasis (i.v. injected melanoma) was determined; drug-induced stimulation of DNA synthesis in C-3 fibrosarcoma cells in vitro was correlated with tumor growth acceleration in vivo. In a mammary carcinogenesis model, the effects of chronic saline, amitriptyline, or fluoxetine administration on the rate and frequency of development of mammary tumors in rats fed dimethylbenzanthracene (DMBA) were compared. Eight of 20 amitriptyline- or fluoxetine-treated mice developed fibrosarcoma tumors by day 5, as compared to none of 20 saline controls (P less than 0.002). Similarly, 20 of 21 DMBA-treated rats receiving the antidepressant drugs developed 33 mammary tumors by week 15 as compared to 5 tumors in 4 of 7 DMBA-treated rats receiving saline (P less than 0.001). For both models, tumor latency decreased 30-40% and, in the DMBA model, tumor frequency increased greater than 2-fold in the antidepressant-treated rats as compared to controls. Stimulation of fibrosarcoma growth in vivo correlated with a corresponding bell-shaped drug-induced increase in DNA synthesis in vitro. While the median time to death from pulmonary metastases did not differ among groups given i.v. injections of melanoma cells, a significant (P less than 0.01) stimulation of growth of s.c. injected melanoma was observed in mice receiving the antidepressants.(ABSTRACT TRUNCATED AT 400 WORDS)

L-histidinol in experimental cancer chemotherapy: improving the selectivity and efficacy of anticancer drugs, eliminating metastatic disease and reversing the multidrug-resistant phenotype.

Human cancer chemotherapy is limited by two major problems: the failure of commonly used anticancer drugs to act against tumor cells in a specific manner and the ability of malignant cells to resist killing by antineoplastic agents. Experimentally, both of these problems can be solved by using L-histidinol in combination with conventional anticancer drugs. A structural analogue of the essential amino acid L-histidine and an inhibitor of protein biosynthesis. L-histidinol improves the selectivity and the efficacy of a variety of cancer drugs in several transplantable murine tumors. Furthermore, L-histidinol circumvents the drug-resistant traits of a variety of cancer cells, including those showing multidrug resistance. This review will summarize these properties of L-histidinol, present new evidence on its ability to increase the vulnerability of both drug-sensitive and drug-resistant human leukemia cells to various anticancer drugs, and show that, in addition to inhibiting protein synthesis, L-histidinol acts as an intracellular histamine antagonist. The establishment of a connection between the latter mechanism and the capacity to modulate anticancer drug action has resulted in a clinical trial in the treatment of human cancer.

Increased therapeutic index of antineoplastic drugs in combination with intracellular histamine antagonists.

L-Histidinol, a protein synthesis inhibitor and structural analogue of L-histidine, has been demonstrated in chemotherapy-treated mice to be cytoprotective to normal stem cells but to enhance cytotoxicity to tumor cells. N,N-Diethyl-2-[4-(phenylmethyl) phenoxy]ethanamine.HCl (DPPE) is an antagonist of recently described microsomal and nuclear intracellular histamine receptors implicated in the mediation of proliferation and modulation of prostaglandin synthesis. DPPE is cytotoxic to tumor cells in vitro and cytoprotective to the gut in vivo. Noting the similar pharmacologic profiles for histidinol and DPPE and the structural resemblance between histidinol and histamine, we tested 1) whether binding to intracellular histamine receptors may be important to the action of histidinol, 2) whether there exists a differential effect of DPPE and histidinol on proliferating normal and transformed or malignant cells, and 3) whether DPPE, like histidinol, protects host cells from the effects of chemotherapy while augmenting tumor cell kill in vivo. It was observed that histidinol does compete at intracellular histamine receptors in isolated microsomes and nuclei, but with significantly lower affinity than DPPE. Nevertheless, for each agent, potency at intracellular histamine receptors correlates with potency to inhibit DNA and protein synthesis, without cytotoxicity, in normal mitogen-stimulated murine lymphocytes and to kill transformed mouse lymphocytes or MCF-7 human breast cancer cells. As demonstrated previously for histidinol (1-2 g/kg), DPPE (4 mg/kg) protected murine bone marrow progenitors from doxorubicin or fluorouracil, while doses of 4-50 mg/kg significantly enhanced the antitumor activity of doxorubicin and daunorubicin in murine models of early cancer. One postulate to explain the effects of intracellular histamine receptor ligands is that intracellular histamine mediates DNA and protein synthesis, possibly through a downward modulation of growth-inhibitory prostaglandin levels. Antagonism of the intracellular action of histamine at intracellular histamine receptors by DPPE or histidinol may result in differential perturbations of growth/eicosanoid metabolism in normal and malignant cells, thus forming the basis of a new approach to chemotherapy.

Effects of L-histidinol on the proliferation and anticancer drug susceptibility of cultured B16f10 melanoma cells.

L-Histidinol, a structural analogue of the essential amino acid L-histidine, is able to enhance the toxicity of a wide variety of anticancer drugs to tumor cells both in vitro and in vivo. In this study, the effects of L-histidinol on the viability, cell cycle traverse and anticancer drug susceptibility of B16f10 melanoma cells in culture have been examined. L-Histidinol inhibited the transit of B16f10 melanoma cells through the cell cycle in a dose-dependent manner. In spite of its capacity to slow cell cycle progression, L-histidinol nevertheless increased the capacity of several antineoplastic agents of varying modes of action to kill B16f10 melanoma cells.

Different effects of L-histidinol and homoharringtonine on 5-fluorouracil and bis-chloroethylnitrosourea activity in a murine model.

A comparison of the effects of L-histidinol and homoharringtonine (HHT) on the activity of 5-fluorouracil (FUra) and bis-chloroehtylnitrosourea (BCNU) in C57/BL mice, without or with disseminated B16f10 melanoma, was carried out. Although L-histidinol and HHT are both protein synthesis inhibitors with apparently identical modes of action, these two compounds had very different effects in the test systems. HHT failed to prevent the body weight lose and subsequent death of C57/BL mice treated with supralethal doses of FUra; it was also unable to prevent the toxicity of FUra for bone marrow cells. In contrast, L-histidinol prevented the weight loss, death and bone marrow damage otherwise resulting from identical doses of FUra. Furthermore, L-histidinol was far more effective than HHT in its ability to improve the management of disseminated B16f10 melanoma in C57/BL mice by BCNU, both in terms of reducing pulmonary foci and extending survival.

Improved treatment of disseminated B16f10 melanoma in mice with anticancer drugs in combination with L-histidinol.

An artificial hematogenous-metastasis model, in which B16f10 melanoma cells injected into the tail veins of C57/BL mice arrest in the lungs and proliferate as discrete pulmonary foci, was employed to examine effects of L-histidinol on the capacity of a number of conventional antineoplastic agents to manage disseminated disease. Treatment responses were evaluated by determining both the number of lung foci and/or by evaluating animal survival. L-Histidinol, on its own, was found to have a significant and dose-dependent capacity to reduce the number of lung foci and to extend survival of animals bearing disseminated B16f10 melanoma. L-Histidinol enhanced the ability of bis-chloroethylnitrosourea, 5-fluorouracil, and 1-beta-D-arabinofuranosulcytosine to reduce the number of lung foci. The latter combinations also gave marked improvements in survival, whether administered 1 or 7 days after the intravenous injection of tumor cells.

3,5-Bis-benzylidene-4-piperidones and related compounds with high activity towards P388 leukemia cells.

3,5-Bis(benzylidene)-4-piperidones and related compounds were prepared and found to have between 100 and 9700 times the activity of N,N'-bis(2-chloroethyl)-N-nitrosourea towards P388 leukemia cells. The shapes of six of these molecules--determined by X-ray crystallography--were compared, but no correlation between the stereochemistry of the molecules or their electronic properties and cytotoxicity was apparent. Molecular modification of the compounds by forming two mono-benzylidene compounds, a related acyclic derivative and an N-acyl compound resulted in diminished but retained high cytotoxicity. Two representative compounds lowered glutathione levels of liver following their intraperitoneal injection into mice. Two quaternary ammonium compounds were shown to bind in the minor groove of DNA, while four related non-quaternary ammonium derivatives did not demonstrate this property. We conclude that the modes of action of these highly cytotoxic compounds include alkylation of cellular thiols and DNA binding, but interference with other biochemical processes is also probably involved.

L-histidinol improves the selectivity and efficacy of alkylating agents and daunomycin in mice with P388 leukaemia.

DBA/2J mice bearing a clonal isolate of the transplantable murine lymphocytic leukaemia line P388 were used to examine the effects of L-histidinol on the antitumour activity of three alkalyating agents (bis-chloroethylnitrosourea (BCNU), cis-diamminedichloroplatinum (II) (cisDDP) and cyclophosphamide) and the antitumour antibiotic daunomycin. Single, combined treatments with L-histidinol and either BCNU or cisDDP, at doses of the alkylating agents which were ineffective when used alone, were completely curative. Dose-response studies showed that L-histidinol conferred dose-dependent, synergistic improvements on the capacities of both BCNU and cisDDP to increase the life-span of DBA/2J mice bearing P388 leukemia. For combinations of L-histidinol and cyclophosphamide or daunomycin, two successive treatments with L-histidinol and drug were required to obtain a significant portion of long-term survivors. Thus, in this model system, the L-histidinol/anticancer drug combination approach for improving experimental cancer chemotherapy can be employed successfully with three alkylating agents and the antitumour antibiotic daunomycin.

Reversal of the multidrug-resistant phenotype of Chinese hamster ovary cells by L-histidinol.

The amino acid analogue L-histidinol reverses the multidrug-resistance (MDR) attribute of the colchicine-resistant (CHR) variant CHRC5, a Chinese hamster ovary cell line that overexpresses a plasma membrane-associated glycoprotein and is resistant to colchicine (CH), daunorubicin, and vinblastine sulfate (VS). The level of cell kill achieved in CHRC5 cells by combinations of L-histidinol and either daunorubicin or CH approached that achieved in AUXB1 parent cells by these two drugs, whereas L-histidinol-VS combinations killed even more CHRC5 cells than VS in the parental line. The capacity of L-histidinol to reverse the MDR phenotype of the CHRC5 line was time and dose dependent and was eliminated by the addition of a twofold molar excess of L-histidine. The reversal of the MDR trait by L-histidinol appears to be independent of the drug uptake mechanism.

Evaluation of Mannich bases of styryl ketones and related hydrazones for activity against P388 leukemia.

A Mannich base namely 4-dimethylaminomethyl-1-phenyl-1-penten-3-one hydrochloride was shown to have far greater activity than 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) towards P388 leukemia cells in vitro. However, the compound was inactive in an in vivo P388 murine screen, and the object of this study was to discern molecular features which conferred in vivo activity. Mannich bases containing electron-attracting substituents in the aryl ring had in vivo potency in contrast to the analogs which had electron-donating groups in the ring. A number of hydrazones of the Mannich bases were prepared as potential prodrugs and did not enhance bioactivity. This observation was probably due to a lack of facile hydrolysis of the hydrazones to the corresponding Mannich bases in vivo since high resolution 1H NMR spectroscopy revealed that representative hydrazones either did not regenerate the ketones or produced them only in minute quantities at pH values normally encountered in living tissues.

Effects of L-histidinol on the susceptibility of P815 mastocytoma cells to selected anticancer drugs in vitro and in DBA/2J mice.

The effects of L-histidinol on the susceptibility of the transplantable murine mast-cell neoplasm P815 mastocytoma to selected anticancer drugs have been evaluated on cells growing in culture and in syngeneic DBA/2J mice. Combinations of L-histidinol and anticancer drugs of either phase specificity [cytarabine (ara-C) and vinblastine sulfate] or cycle specificity [5-fluorouracil (FUra) and methotrexate] had diverse effects on cultured mastocytoma cells as scored by clonogenic cell survival assays. Flow cytometric analysis of randomly proliferating P815 mastocytoma cells revealed that although exposure to L-histidinol did not preclude cells from traversing the cell cycle, the analogue nonetheless conferred a dose-dependent and apparently nonspecific delay of cell cycle transit. DBA/2J mice bearing intraperitoneal P815 mastocytoma cells were used to evaluate the in vivo efficacy of L-histidinol-ara-C and of L-histidinol-FUra combinations. Quantitative cell survival assays of murine bone marrow cells and of clonogenic tumor cells obtained from treated animals demonstrated that L-histidinol eliminated the bone marrow toxicity otherwise attending the use of the drugs ara-C and FUra. Simultaneously, the inclusion of L-histidinol provided a statistically significant increase in the capacity of these two anticancer drugs to eradicate intraperitoneal mastocytoma cells.