Antitumor activity in mice was observed for the oxime of the previously reported ethyl [6-amino-4-[(1-methyl-2-phenyl-2-oxoethyl)amino]-5-nitropyridin -2-yl] carbamate (8) and several related compounds. These compounds are precursors of the active ethyl pyrido[3,4-b]pyrazin-7-ylcarbamates (e.g., 4), which are potent antimitotic agents. In the 5-nitropyridine series overall biological activity was reduced by replacement of the oxime moiety with a keto or alcohol group and by replacement of the 1-methyl group of the side chain with hydrogen. Reduction of the nitro group of the 5-nitropyridines containing an alcohol in the side chain to the corresponding 5-aminopyridines increased biological activity. Preliminary studies showed that the 5-nitropyridine oximes were considerably less potent than the pyridopyrazines as antimitotic agents and that the former are apparently not converted to the latter in vivo. The inhibition of the incorporation of pyrimidine nucleosides into DNA and RNA was identified as another possible mode of action of the 5-nitropyridine oximes.

The synthesis of ring analogues and derivatives of the S isomer of ethyl [5-amino-1,2-dihydro-2-methyl-3-phenylpyrido[3,4-b]pyrazin-7 - yl[carbamate, (S)-1, a potent antimitotic agent with anticancer activity, was directed toward the determination of the contribution of several structural features of this compound to biological activity. Replacement of the 5-amino with a 5(6H)-oxo group and either transposing the 6-ring nitrogen to or incorporation of a ring nitrogen at the 8-position caused a significant decrease in in vitro activity and destroyed in vivo activity. Although in vivo cytotoxicity was reduced, in vitro activity at higher doses relative to (S)-1 was retained by replacement of the 5-amino group with hydrogen and by expansion of the 1,2-dihydropyrazine to give a dihydro-1,4-diazepine ring.

Metabolism studies with ethyl [5-amino-1,2-dihydro-2-methyl-3-phenylpyrido[3,4-b]pyrazin-7 - yl]carbamate (1) in mice were reported previously to give a hydroxylated metabolite, which was methylated to give a methoxy derivative. The metabolite and its derivatives were considered to be 4-(substituted)phenyl compounds, which have been confirmed by the synthesis of the [1,2-dihydro-3-(4-hydroxyphenyl)- and [1,2-dihydro-3-(4-methoxyphenyl)pyrido[3,4-b]-pyrazin-7-yl]carbama tes (17 and 16). Both the S- and R-isomers of 17 are active in several biological systems, but the S-isomer is more potent then the R-isomer. The difference in activity between the S- and R-isomers of 17 is similar with that observed for S- and R-isomers of 1. As model reactions, several O-substituted derivatives were prepared by alkylation of (RS)-17 with benzyl chloride and condensation of (RS)-17 with butyl isocyanate and (S)-17 with 2-chloroethyl isocyanate.

The reaction of ethyl (6-amino-4-chloro-5-nitropyridin-2-yl)carbamate (2) with alpha-amino ketone oximes gave 4-[(2-oxoethyl)amino]pyridine oximes 3, which were reductively cyclized to give a series of ethyl (1,2-dihydro-pyrido[3,4-b]pyrazin-7-yl)carbamates (6). In another approach, alpha-nitro ketones, alpha-oximino ketones, and alpha-nitro alcohols were reduced to give alpha-amino alcohols, which were reacted with 2 to give 4-[(2-hydroxyethyl)amino]pyridines (5). Oxidation of these alcohols with the chromium trioxide-pyridine reagent gave the corresponding ketones (4), which were also reductively cyclized to give 6. Structure-activity relationship studies indicated that alterations at the 2- and 3-positions of the pyrazine ring of 6 had a significant effect on cytotoxicity and the inhibition of mitosis in cultured lymphoid leukemia L1210 cells. Compounds that exhibited in vitro cytotoxicities at less than 1 nM showed the same level of in vivo activity, whereas the less potent compounds showed wide variations in their in vivo activity.

Cyclization of ethyl 5,6-diamino-4-hydrazinopyridin-2-ylcarbamate (10) with a mixture of CS2 and Et3N in dimethylacetamide gave mainly ethyl 1,4-diamino-2(3H)-thioxoimidazo[4,5-c]pyridin-6-ylcarbamate (15), whereas, in the absence of dimethylacetamide, a double cyclization gave mainly ethyl 5-amino-2(1H)-4-dithioxodiimidazo-[4,5-b:5,4-c]pyridin-7-ylcarb amate (16). Cyclization of the benzylidenehydrazino derivative (6) of 10 with either CS2-Et3N or (EtO)3CH-HCl gave 1-(benzylideneamino)imidazo[4,5-c]pyridines 11 and 7 as major products and 7-(benzylidenehydrazino)imidazo[4,5-b]pyridines 12 and 8 as minor products. Dethiolation of 11 to give 7 and of 12 to give 8 was effected with excess Raney nickel in refluxing ethanol. The benzylidene group of 11 was removed with hydrazine in ethanolic HCl to give 15. This key compound was condensed with benzaldehydes to give 1-benzylideneamino derivatives (20, 21) and alkylated with benzyl halides to give 2-benzylthio derivatives (24-26). In addition, cyclization of ethyl 5,6-diamino-4-(benzylidene-1-methylhydrazino)pyridin-2-ylcarbam ate (30) with (EtO)3CH provided a method for the synthesis of an imidazo[4,5-c]- and -[4,5-b]pyridines gave compounds that inhibited proliferation of growth and caused mitotic arrest against lymphoid leukemia L1210 at micromolar concentrations. However, the more active in vitro compounds (7, 8, 24-26) gave only borderline activity in mice against lymphocytic leukemia P388.

Ring analogues and derivatives of the 1,2-dihydropyrido[3,4-b]pyrazin-7-ylcarbamates (e.g., 29), antimitotic agents with antitumor activity, were prepared in the search for compounds with greater selectivity. Methods were developed for the conversion of substituted benzoic acids (1-4) to give benzopyrazines (12-16 and 21) and of substituted pyridin-2-carbamates (23, 38, and 41) to give 2-aminopyrido[3,4-b]pyrazin-7-ylcarbamates (32 and 36) and pyrido[3,4-e]-as-triazin-7-ylcarbamates (47 and 50). In vitro evaluation indicated that activity was reduced by removal of the pyridine ring nitrogen of 29 to give 14 and was destroyed by increasing the basicity of the pyrazine ring of 29 to give 32 and 47.

Racemic ethyl 5-amino-1,2-dihydro-2-methyl-3-phenylpyrido[3,4-b]pyrazine-7- carbamate (1a) has shown antitumor activity in a variety of in vivo experiments. The preparation of the R and S isomers gave compounds with significant differences in potency in several biological tests.

The ethyl (1,2-dihydropyrido[3,4-b]pyrazin-7-yl)carbamates have been reported to bind with cellular tubulin, to produce an accumulation of cells at mitosis, and to exhibit cytotoxic activity against experimental neoplasms in mice. Studies on the disposition of ethyl (5-amino-1,2-dihydro-2-methyl-3-phenylpyrido[3,4-b]pyrazin-7 -yl)carbamate (8) in mice showed that one metabolite was formed by cleavage of the ethyl carbamate moiety. Analogues with alterations in the carbamate group were prepared by transformations at the carbamate of 8, by reductive cyclization of nitropyridine intermediates, and by hydride reduction of the ring of heteroaromatic compounds. In vitro and in vivo evaluations of analogues indicated that a carbamate group was required for activity. No significant change in activity was observed when ethyl was replaced by methyl. However, activity was reduced when ethyl was replaced with bulky aliphatic groups and when ethoxy was replaced with a methylamino group. Also, the activity of 8 was decreased by acetylation of the 5-amino group and was destroyed by substitution of an amino group at the 8-position.

5-[[N-[(Ethoxycarbonyl)alkyl]amino]carbonyl] (6-9) and the corresponding aminothiocarbonyl (12-15) derivatives of 5,6,7,8-tetrahydrofolic acid were prepared as multisubstrate analogues of the substrate--cofactor adduct in the reactions catalyzed by the folate-mediated one-carbon transfer reactions. Evaluation in vitro showed that 7 (alkyl = hexyl) was cytotoxic to H.Ep.-2 cells (ED50, 4 microM) but noncytotoxic to proliferating L1210 cells. No activity was observed for 7 against the P388 leukemia in mice.

The 1,2-dihydropyrido[3,4-b]pyrazines (1) are mitotic inhibitors with significant antitumor activity in mice. Also, the active imidazo[4,5-b]pyridine 3 was shown to cause the accumulation of cells at mitosis. Routes were developed for the synthesis of congeners of 3 by cyclization of 4-(substituted amino)-5,6-diaminopyridines with ethyl orthoformate. Oxidative cyclization of either 4,5- or 5,6-diaminopyridines with aryl aldehydes produced the [4,5-c] and [4,5-b] imidazopyridine ring systems, respectively. The latter reaction with 6-(substituted amino)-4,5-diaminopyridines gave imidazo[4,5-c]pyridine ring analogues of 1. Biological studies indicated that the target compounds were less active than 1 and 3.

Several properties of four 1-deaza-7,8-dihydropteridines were compared with those of each other and with those of colchicine, nocodazole, podophyllotoxin, and vincristine. Compound NSC 370147 was more active than the other compounds of this type with respect to inhibition of proliferation of cultured L1210 cells and to increase of the mitotic index. On an equimolar basis it was more active than two of the 1-deaza-7,8-dihydropteridines, colchicine, and nocodazole and was comparable to podophyllotoxin and vincristine in inhibiting the polymerization of partially purified pig brain tubulin. All four of the 1-deaza-7,8-dihydropteridines caused decreases in the extent of binding of [3H]colchicine to partially purified tubulin and enhanced the binding of [3H]vincristine to the tubulin. Emphasis in further testing was placed upon NSC 370147, because it is easier to synthesize and is more stable than some of the other compounds of this type and because its greater solubility in water facilitates its formulation for therapeutic administration. Compound NSC 370147 inhibited competitively the binding of [3H]colchicine to purified tubulin and enhanced slightly the binding of [3H]vincristine to tubulin. It was also synergistic with vincristine in killing cultured L1210 cells and in increasing the life-spans of mice bearing P388 leukemia. It is suggested that it would be worthwhile to evaluate combinations of NSC 370147 and vincristine in tests with other experimental neoplasms.