Alkylformamides as inducers of tumour cell differentiation--a mini-review

The induction of terminal differentiation in tumour cells represents a possible therapeutic strategy for treating cancer. The alkylformamides are 1 group of experimental compounds which have been shown to induce terminal differentiation in human HL-60 leukemia and murine Friend erythroleukemia cells in vitro. Their mechanism of action is unknown. Dimethylformamide has been used as a model inducer in carcinoma and fibroblastic models. Analysis of the relationship between structure and inducing activity of the alkylformamides in vitro reveals that no specificity of structure exists and that their properties as inducers of terminal differentiation extend to related compounds, e.g. the alkylacetamides and alkylureas. This is in contrast to the marked specificity of N-methylformamide (NMF) as an in vivo antitumour agent. The potency of these compounds as inducers of differentiation is predictable and correlated with their molecular weight. High concentrations of NMF are required to induce differentiation in vitro and these concentrations are not achievable in vivo. However, while NMF is unlikely to be a useful inducer in vivo many of its higher MW analogues are very much more potent as inducers in vitro and yet no more toxic (to the host) in vivo. Some of these (e.g. tetramethylurea or 1,3-dimethylurea) may be capable of achieving inducing concentrations in vivo.

Correlation between the molecular weight and potency of polar compounds which induce the differentiation of HL-60 human promyelocytic leukemia cells

Structure-activity studies of a series of polar organic compounds, including N,N-dimethylformamide, N-methylformamide, and related ureas and acetamides, were performed with regard to their ability to promote the terminal differentiation of the human promyelocytic leukemia cell line HL-60 to granulocyte-like cells. Functional and morphological criteria were used to assess the percentage of differentiated cells which arose from their continuous incubation with different concentrations of each agent over a period of 96 h. All of the alkylformamides, alkylacetamides, and alkylureas tested were found to induce differentiation, regardless of structure. Inspection of the results showed that there was a linear relationship (r = -0.937) between the molecular weight of each compound and the logarithm of the concentration which was required to bring about the differentiation of the greatest number of cells, while viability was generally maintained at greater than 85%. Once established, this relationship was used to predict the potency of several polar solvents which were structurally unrelated to the formamides. For example, methanol, ethanol, and acetone were all inducers of differentiation with a potency predictable from their molecular weight alone. The terminal differentiation induced by all of the compounds was only accomplished by cells which were capable of replication prior to differentiation. At concentrations which prevented a single replication and brought about a fall in cell viability over 96 h, no differentiation was observed. A correlation was observed between the molecular weight of each compound and the logarithm of its concentration to bring about cytotoxicity without differentiation (r = -0.935), and the line was almost parallel to that defining the concentration required for optimal differentiation (slope values of -0.02126 and -0.02288). A poorer (r = -0.6654) correlation was found between the logarithm of the octanol-water partition coefficient and the logarithm of the concentration required for optimal differentiation, when the data for 12 of the polar organic compounds were analyzed. The results suggest that no special structural requirements are necessary for the alkylformamides, -acetamides, -ureas, and related compounds to induce the terminal differentiation of HL-60 cells to granulocyte-like cells, but that the activity of each compound could be predicted from their molecular weight. The concentrations required to induce differentiation were marginally lower than those which were cytostatic or cytotoxic, which suggested that a toxic threat to the cells was sufficient to induce differentiation.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural studies on bioactive compounds. 4. A structure-antitumor activity study on analogues of N-methylformamide

A series of derivatives of N-methylformamide (NMF), an experimental antitumor agent, has been prepared, having the general formula R3C(X)NR1R2 where R1 = H, CH3, CD3, CH2CF3, CH2CH2Cl, cyclopropyl, C2H5, CH2OH, CH2OR, CH2N(CH3)2; R2 = H, CH3; R3 = H, CF3, CCl3, CH3, Ph, NHCH3, N(CH3)2; and X = O, S, NH. A further short series of "push-pull" olefins of the general formula R1R2C = CHNR3R4 has been synthesized where R1 = H, CH3 and R2 = H, NO2, CN, CHO, CH3 and R3 = H and R4 = H, CH3, morpholino. These compounds have been tested for activity against the M5076 ovarian sarcoma and the TLX5 lymphoma in mice. NMF was by far the most potent agent of both series with activity against both tumors. Some other compounds showed weak activity, but there is a rigorous structural requirement for activity and most analogues were inactive. Certain members of the series exist as equilibrium mixtures of rotamers about the amide or pro-amide bonds as shown by NMR.

The antitumour effect and toxicity of cis-platinum and N-methylformamide in combination

N-Methylformamide (NMF), currently undergoing phase II clinical evaluation for the treatment of cancer, and the established antitumour agent cis-platinum (CDDP) have nonoverlapping toxicities, with the exception of gastrointestinal side effects. The major target organs for the toxicities of the compounds are the liver (NMF) and the kidney (CDDP). Furthermore, NMF is nonleukopenic. In view of this, and of recent evidence that NMF enhances the cytotoxic effect of CDDP in vitro the activity of NMF and CDDP against the M5076 sarcoma implanted in mice was investigated, together with the various toxicities in mice and rats. The antitumour effect of NMF in combination with CDDP was additive, but NMF did not alter the leukopenia produced by CDDP in the tumour-bearing mice. CDDP produced only a minimal increase in the hepatotoxicity of NMF in mice, and NMF did not augment the nephrotoxicity of CDDP in rats (except for a small effect on calcium excretion). The results support suggestions that clinical evaluation of combination chemotherapy with NMF and CDDP is warranted.

Experimental antitumor activity against murine tumor model systems of 8-carbamoyl-3-(2-chloroethyl)imidazo[5,1-d]-1,2,3,5-tetrazin-4(3 H)-one (mitozolomide), a novel broad-spectrum agent

8-Carbamoyl-3-(2-chloroethyl)imidazo[5,1-d]-1,2,3,5-tetrazin-4(3H) -one (mitozolomide) demonstrates curative action against a range of murine tumor model systems. At single doses of between 20 and 40 mg/kg, the latter of which approximates the 10% lethal dose value in mice, the compound elicited cures against the L1210 and P388 leukemias irrespective of the route of tumor and/or drug administration; in these tests, animals receiving 10(5) cells i.p. survived greater than 60 days after treatment. Potent effects were also observed against the TLX5 lymphoma (s.c.) and B16 melanoma (i.p.). In other experiments, 7 of 10 animals implanted with 2 X 10(5) Lewis lung carcinoma cells survived greater than 60 days while 10 of 10 animals survived greater than 60 days after implantation of the Colon 26 tumor. Potent inhibition of the solid tumor models was also observed with complete cures of the Colon 38, M5076 sarcoma, and ADJ/PC6A plasmacytoma. In cross-resistance studies, the compound was ineffective against an L1210 leukemia made resistant to 1,3-bis(2-chloroethyl)-1-nitrosourea and against a TLX5 lymphoma resistant to dimethyltriazenes but cured animals bearing the L1210 leukemia with derived resistance to cyclophosphamide.

N-Methylformamide (NSC 3051): a potential candidate for combination chemotherapy

N-Methylformamide (NMF) was found to be non-toxic to the bone marrow as reflected in the absence of leukopenia in mice, even when the marrow had been compromised by prior administration of cyclophosphamide. Thus recovery from the leukopenic nadir after 160 mg/kg of cyclophosphamide was unaffected by 200 mg/kg X 10 of NMF. This combination, given to animals bearing the M5076 sarcoma, proved to have an additive antitumour effect as measured by tumour growth delay and was superior to the antitumour effect of two doses of cyclophosphamide, a regime which prolonged the leukopenia. Furthermore, the hepatotoxicity of NMF was not augmented by the addition of cyclophosphamide. When hepatotoxicity was induced in BALB/c mice bearing the NMF-resistant ADJ/PC6A plasmacytoma, cyclophosphamide fully maintained its antitumour effect. The results show NMF to be a highly specific antiproliferative agent with potential for use in the therapy of patients with a compromised bone marrow and/or in combination chemotherapy.

Studies on the toxicity of the antitumour agent N-methylformamide in mice

Aspects of the toxicology of N-methylformamide (NMF), an investigational antitumour agent, were studied in mice. After injection of NMF at its LD10 (800 mg/kg) dosage the total peripheral white blood cell and platelet counts were unchanged in BALB/c mice. A mild granulocytosis was seen in this strain after administration of the LD50 (2300 mg/kg) dosage. Plasma activity of the enzyme sorbitol dehydrogenase in BDF1 mice was markedly increased after either a single injection of not less than 800 mg/kg or a chronic treatment of not less than 400 mg/kg/day over 5 days indicating the drug to be hepatotoxic. Plasma activities of L-alanine and L-aspartate aminotransferases were also increased after the chronic treatment. Chronic administration of NMF was less hepatotoxic than single dose administration of the same total dose and also increased the antitumour efficacy of NMF against the M5076 sarcoma. These results indicate that the maximum therapeutic benefit of NMF might be obtained by the use of chronic schedules and that the drug is not myelosuppressive.

The chemosensitivity of a new experimental model--the M5076 reticulum cell sarcoma

The M5076 reticulum cell sarcoma is a murine tumour of potential value in experimental chemotherapy. Experiments were conducted to ascertain the growth characteristics and chemosensitivity of this neoplasm in the BDF1 mouse. The intramuscular tumour proved to be responsive to the alkylating agents, nitrosoureas, procarbazine, DTIC and treosulphan, yet insensitive to the antimetabolites and only weakly responsive to adriamycin. Analogues of the antitumour agents hexamethylmelamine and N-methylformamide were tested against this neoplasm. The patterns of activity determined for these analogues against this tumour were identical to those previously reported against other model systems.

Studies of the mode of action of antitumour triazenes and triazines-V. The correlation of the in vitro cytotoxicity and in vivo antitumour activity of hexamethylmelamine analogues with their metabolism

Experiments were conducted to ascertain whether the antitumour activity of hexamethylmelamine analogues correlated with their in vitro cytotoxicity and metabolism. Two analogues, namely pentamethylmelamine (PMM) and 2,2,4,4-tetramethylmelamine (TMM), and hexamethylmelamine (HMM) itself were shown to be active towards the murine ADJ/PC6A (PC6) plasmacytoma; another three, 2-chloro-4,6-bis(dimethylamino)-1,3,5-triazine (CBDT), 2,4-bis-(dimethylamino)-6-hydrazino-1, 3,5-triazine (HBDT) and 2,4,6-trimethylmelamine (TriMM) were inactive against the same tumour. The cytotoxicity of these compounds was examined against a PC6 tumour cell line in vitro. In the absence of liver microsomal activation only CBDT proved to be significantly cytotoxic at a concentration of 5 mM. In the presence of murine liver microsomes the three active antitumour agents were all cytotoxic at this concentration whereas HBDT and TriMM remained non-toxic. The degree of cytotoxicity correlated with the extent of metabolism for these analogues. The products of biotransformation of these compounds were stable precursors of formaldehyde (presumably N-hydroxymethyl intermediates) (FP) rather than formaldehyde itself. After injection of these 6 compounds to Balb/c mice the levels of FP generated in the plasma were markedly greater for the three active antitumour agents than for the inactive analogs. No free formaldehyde was detected in the plasma after administration of any of the compounds. These results suggest that for these compounds in vitro cytotoxicity correlates with in vitro biotransformation and their antitumour activity correlates with plasma levels of FP generated by metabolism in vivo.

The formation and metabolism of N-hydroxymethyl compounds--IV. Cytotoxicity and antitumour activity of N-hydroxymethylformamide, a putative metabolite of N-methylformamide (NSC 3051)

Experiments were conducted to test the hypothesis that N-hydroxymethylformamide (HMF) is the active metabolite of the antitumour agent N-methylformamide (NMF). In an in vitro bioassay against the TLX5 lymphoma HMF was more cytotoxic than NMF; this cytotoxicity was abolished by preincubating the TLX5 cells with semicarbazide, a formaldehyde trapping agent. Similarly, the inhibition of incorporation of radiolabelled thymidine, uridine, formate and leucine into TLX5 cells elicited by HMF was eliminated by preincubation of the cells with semicarbazide. HMF is considerably less toxic to tumour-bearing BDF1 mice than NMF and, unlike NMF, does not reduce hepatic glutathione levels in vivo. HMF has no inhibitory activity against the TLX5 lymphoma or the Sarcoma 180 in mice in vivo and only marginal activity against the M5076 reticulum cell sarcoma; these tumours are highly sensitive to NMF. However, like NMF, HMF inhibits growth of the human mammary tumour MX-1 implanted in the subrenal capsule of mice.

N-methylformamide: antitumour activity and metabolism in mice

The antitumour activities of N-methylformamide, N-ethylformamide and formamide against a number of murine tumours in vivo (Sarcoma 180, M5076 ovarian sarcoma and TLX5 lymphoma) have been estimated. In all cases N-methyl-formamide had significant activity, formamide had marginal or no activity and N-ethylformamide had no significant activity. N-methylformamide and N-ethylformamide were equitoxic to the TLX5 lymphoma in vitro. Formamide was found as a metabolite in the plasma and urine of animals given N-methylformamide and N-ethylformamide, but excretion profiles do not support the hypothesis that formamide is an active antitumour species formed from N-alkylformamides. No appreciable metabolism of N-methylformamide occurred under a variety of conditions with liver preparations in vitro. N-methylformamide, but not N-ethylformamide or formamide, reduced liver soluble non-protein thiols by 59.8% 1 h after administration of an effective antitumour dose.

Studies of the mode of action of antitumour triazenes and triazines-III. Metabolism studies on hexamethylmelamine

These is good evidence that the antitumour agent hexamethylmelamine (HMM) undergoes oxidative metabolic activation which might occur in the liver and/or extrahepatically. The hepatic microsomal N-methylmelamine metabolizing enzymes were investigated in mice and exhibited different affinities for different melamine derivatives. The apparent Km values are 0.09 mM for HMM, 0.23 mM for pentamethylmelamine, 0.91 mM for 2,2,4,6-tetramethylmelamine and 1.7 mM for trimethylmelamine. HMM inhibited its own metabolism in vitro at substrate concentrations greater than 0.05 mM. Its hepatic microsomal N-demethylation rate was reduced when the mice were pretreated with the hepatic glutathione depleting agent methyliodide. Injection of hexaethylmelamine, a derivative of HMM without antineoplastic properties against the M5076 sarcoma in mice, lead to plasma concentrations of drug and metabolite pentaethylmelamine which were only a fraction of the drug and metabolite levels achieved after a similar dose of HMM.

The stereochemical course of amino acid activation by methionyl- and tyrosyl-tRNA synthetases

Stereochemical analysis has long been recognised as a powerful tool for elucidating the mechanisms of chemical and enzyme-catalysed reactions. Although much is known about the stereochemical course of reactions at saturated carbon, phosphate and thiophosphate esters whose ligands to phosphorus are also tetrahedrally disposed, are capable in principle of revealing sterochemical information about events at the active site of enzymes that transform such substrates. Nucleotidyl transferases are a group of enzymes which in general selectively use one of the diastereoisomers of a nucleoside 5'(1-thiotriphosphate), such as isomers A and B of adenosine 5'(1-thiotriphosphate), designated ATP alpha S-A and ATP alpha S-B, and allow investigation of the stereochemical course of nucleotidyl transfer. We have developed a simple method based on 31P nuclear magnetic resonance spectroscopy for determining the stereochemical course of these reactions, and using this method show here that the nucleotidyl transfer step in two aminoacyl-tRNA synthetases from Escherichia coli occurs with inversion of configuration at phosphorus. These observations greatly constrain the mechanistic possibilities for these enzymes, and are interpreted most simply as a direct 'in line' transfer from ATP to the amino acid.