Clinical studies with MTA.

MTA (LY231514), a multi-targeted antifolate, is a classical antifolate undergoing intracellular polyglutamation. Polyglutamated MTA is a potent thymidylate synthase (TS) inhibitor and inhibits other folate-dependent enzymes, including dihydrofolate reductase and glycinamide ribonucleotide formyl transferase. Multifocal antifolates may overcome antifolate resistance, but it is not known whether the anti-tumour activity of MTA depends on its TS inhibition, its primary locus of action, or whether other loci contribute. MTA was examined in three phase I trials using different schedules: a 10-min i.v. infusion given once every 3 weeks, once weekly for 4 weeks every 6 weeks or daily for 5 days every 3 weeks. Dose-limiting toxicities were neutropenia and thrombocytopenia. Other consistently seen side-effects, which were manageable, included mucositis, skin rashes and transient elevations of transaminases. Toxicity was highly schedule dependent: the recommended dose for the 3-weekly schedule (600 mg m(-2)) was 30 times that for the daily x 5 schedule (4 mg m(-2)day(-1)). The 3-weekly dosing schedule was chosen for phase II evaluation. Phase II trials are underway to investigate the activity and toxicity of MTA in several tumour types, including colorectal, pancreas, breast, bladder and non-small-cell lung cancer (NSCLC) Further phase I trials will investigate MTA in combination with other agents, including gemcitabine, cisplatin, 5-fluorouracil and folate. Preliminary phase II trials results are encouraging; responses were seen in colorectal, pancreas, NSCLC and breast cancer.

Role of the extracellular matrix on the growth and differentiated phenotype of murine colonic adenocarcinoma cells in vitro.

The growth and differentiation characteristics of MAC 15 murine adenocarcinoma cells, derived from routine passage in vivo for growth in vitro on a plastic substrate (MAC15j cells), were compared under conditions in which the cells were seeded onto a substrate of type-I collagen which was either attached to plastic or was released to float free in medium. Cells grown on a plastic substrate consisted of a heterogeneous, largely anaplastic population with a putative enterocytic morphology but with no evidence of junctional complexes or cell polarity typical of an epithelial phenotype. MAC 15j cells from cultures grown on a plastic substrate reestablished a moderate to well-defined degree of differentiation when transplanted back into NMRI mice. When MAC 15j cells were seeded from plastic onto type-I collagen, either attached to plastic or free-floating, tight junctional complexes were formed and the cells began to attain a more recognizable, columnar and polarised epithelial morphology. Cells grown on a type-I collagen gel which was free-floating showed a selective expression of alkaline phosphatase at the apical surfaces of approximately 10% of the cells. This expression was detectable by electron microscope histochemistry but could not be detected biochemically. Treatment of MAC 15j cells grown on a released collagen matrix with tetramethyl-urea (20mM) accelerated the expression of alkaline phosphatase activity at the apical surface as detected by microscopy.

High uptake of RSU 1069 and its analogues melanotic melanomas.

RSU 1069 and RSU 1164 are electron affinic agents that contain a nitro group together with a weakly basic alkylating aziridine moiety, and they represent lead compounds in the development of dual-function, bioreductive, hypoxic cell radiosensitizers. We studied the pharmacokinetics of these drugs in mice carrying KHT sarcoma. Lewis lung carcinoma, and B16 melanoma. Following an i.p. dose of 80 mg/kg, absorption was rapid and the elimination t1/2 was in the region of 30 min for both agents. Maximal tumour levels were 91, 16 and 19 microgram/ml for RSU 1069 and 109, 26 and 28 microgram/ml for RSU 1164 in. the B16, KHT and Lewis lung tumours, respectively. In B16 melanoma these levels corresponded to tumour:plasma ratios of 3.8 for RSU 1069 and 3.7 for RSU 1164. Cellular uptake of RSU 1069, RSU 1164 and a related compound, RB 7040, was measured in vitro as a function of extracellular pH. Melanotic cells from both B16 melanoma and HX118, a human tumour xenograft, showed substantially greater accumulation of these weakly basic sensitizers than any other cell type examined. Ratios of intra-:extracellular concentration (Ci/Ce) for RSU 1069 were around unity and independent of pH for Lewis lung cells and HX34 amelanotic melanoma cells, whereas ratios of up to 3 and 5 were obtained in B16 and HX118 cells, respectively. The highest measured value of Ci/Ce was 15 for RSU 1164 in HX118 cells at pH 8.4; this compares with a ratio of 1.5 for HX34 cells at the same pH. These studies indicate that the high levels of uptake of the weakly basic sensitizers into melanotic melanoma in vivo is a cell-mediated phenomenon and may be due to a lower average intracellular pH in the melanotic cells.

Radiosensitization by the 2,4-dinitro-5-aziridinyl benzamide CB 1954: a structure/activity study.

CB 1954 (2,4-dinitro-5-aziridinyl benzamide) is a radiosensitizer which is up to 10 times more efficient in vitro than would be predicted on the basis of its electron affinity. In order to determine the contribution of the various functional groups comprising the molecule to overall sensitizing efficiency, nine structural analogues have been studied. The redox potential, E7(1), and sensitizing efficiency, C1.6, were obtained for each compound. The value of C1.6 depends on both redox potential and the magnitude of an additional component defined by C1.6/C1.6, where C1.6 is derived from a structure/activity relationship (Adams et al. 1979 b, Wardman 1982) described by the equation: log (C1.6/mol dm-3) = (6.96 +/- 0.22) + (9.54 +/- 0.56)E7(1)V. The magnitude of C1.6/C1.6 for CB 1954 and its analogues depends on alkyl substitution of the amide, the presence/absence and position of the nitro groups and is independent of the presence of the aziridine group. Holding cells in the presence of the drug post-irradiation marginally enhanced sensitization by CB 1954, CB 10-107 and by CB 10-092 but the largest effect was seen with the mononitro compound CB 7060 which also has a value of 26 for C1.6/C1.6. This compound was also interesting in that when combined with 2-phenyl-4(5)amino-5(4)-imidazole carboxamide (phenyl AIC) an enhancement of sensitization was obtained. In contrast, phenyl AIC protected against radiosensitization by CB 1954. Taken together, the data suggest that multiple mechanisms of radiosensitization may contribute to the abnormal radiosensitizing efficiency of CB 1954 and its analogues. This has implications for the further design and development of novel radiosensitizing drugs.

Chemosensitization by monofunctional alkylating agents.

The chemosensitizing ability of model monofunctional alkylating agents with known DNA base alkylating characteristics, that is, methylmethanesulphonate (MMS), ethyl methanesulphonate (EMS) and N-methyl-N-nitrosoguanidine (MNNG) have been investigated. Whereas the alkyl sulphonates chemosensitize V79 cells to cisplatin and melphalan, MNNG does not. The dose response curves show shoulder removal. Drug scheduling and thiourea post-treatment experiments indicate that the effect is likely to be on the initial alkylation rather than completion of crosslink formation from an initial adduct.

Analogues of RSU-1069: radiosensitization and toxicity in vitro and in vivo.

A series of nitroimidazoles containing aziridine and alkyl-substituted aziridine functions has been synthesized. The 2-nitroimidazole compounds examined all show greater radiosensitizing efficiency in vitro than misonidazole. The 4- and 5-nitroimidazole analogues are also more efficient than equivalent compounds which do not contain the alkylating aziridine moiety. All the compounds show increased toxicity towards hypoxic cells relative to aerobic cells, but this toxicity is reduced by alkyl-substitution of the aziridine ring. In vivo toxicity can also be reduced by modification of the aziridine function, but such alterations appear to have less effect upon the high radiosensitizing efficiency of these compounds in vivo. As a consequence of this study, compounds of potentially improved therapeutic utility compared to RSU-1069, the first reported member of this class, have been identified.

Studies on the mechanisms of the radiosensitizing and cytotoxic properties of RSU-1069 and its analogues.

RSU 1069 is a substantially more efficient sensitizer than misonidazole when hypoxic Chinese hamster V79 cells are irradiated in vitro at room temperature; such that for 0.5 mmol dm-3 sensitizer an ER of 3.0 is obtained for RSU 1069 whereas an ER of only 1.6 is obtained for misonidazole. However, when irradiation is done at 4 degrees C, the radiosensitization caused by RSU 1069 is reduced to a level close to that obtained with misonidazole, the action remaining unaltered at the lower temperature. This temperature dependent component of sensitization for RSU 1069 suggests the involvement of a slow biochemical process that has an appreciable activation energy. The RSU 1069 analogue RB 7040 is a more efficient radiosensitizer than RSU 1069 particularly at lower concentrations. This compound has a pKa value of 8.45, in contrast to that of RSU 1069 which is 6.04. Weak bases with pKa values in excess of average intra-cellular pH can be taken up preferentially into cells from medium at pH 7.4. It is shown that RB 7040 has a 4 X higher intracellular concentration than RSU 1069 for a similar extracellular concentration. This will explain, at least in part, the greater sensitizing efficiency of this compound when compared to RSU 1069 in vitro.

The differential cytotoxicity of RSU 1069: cell survival studies indicating interaction with DNA as a possible mode of action.

The hypoxic cell radiosensitizer RSU 1069 (1-(2-nitro-1-imidazolyl)-3-(1-aziridinyl)-2-propanol) shows, on a concentration basis, a 100-fold greater toxicity towards hypoxic relative to aerobic cells. This toxicity is substantially greater than that of misonidazole, a compound of similar electron affinity. Reductive processes are important for hypoxic toxicity; this is demonstrated by the fact that misonidazole, in excess, can protect against the hypoxic but not aerobic toxicity of RSU 1069. The importance of the interaction of RSU 1069 with DNA, suggested initially by molecular studies, is supported by the fact that cells containing 5-bromodeoxyuridine (5-BUdR) incorporated into their DNA show greater sensitivity towards the lethal effects of RSU 1069 both in air and nitrogen, compared to cells not treated with 5-BUdR. Experiments with RSU 1069 and 3-aminobenzamide (3-AB) show the latter compound to potentiate aerobic toxicity, consistent with monofunctional alkylation by RSU 1069. In contrast, 3-AB has no effect on the hypoxic cytotoxicity of RSU 1069, which would be predicted if RSU 1069 is functioning as a bifunctional agent under these conditions. It is our contention that in air, RSU 1069 functions as a typical monofunctional alkylating agent, presumably due to the presence of the aziridine group whereas, in hypoxia, reduction of the nitro group provides an additional alkylating species, converting the compound into a bifunctional agent.

Chemopotentiation by CB 1954: the importance of postincubations and the possible involvement of poly(ADP-ribosylation).

CB 1954 potentiates the cytotoxic action of the bifunctional alkylating agent melphalan (L-PAM). In vitro, this potentiation does not require the preincubation in hypoxia normally needed for other nitroaromatic compounds such as misonidazole. Chemopotentiation is observed when cells are held in CB 1954 in air after treatment with L-PAM. This may reflect an inhibition of DNA repair process(es). Structural considerations suggested that CB 1954 might be acting as an inhibitor of poly(ADP-ribosylation). However, an inhibition of the drop in NAD levels consequent on exposure to melphalan was not obtained. Furthermore, unlike the known poly(ADP-ribose) inhibitor, 3-aminobenzamide, CB 1954 does not potentiate the cytotoxicity of the monofunctional alkylator N-methyl-N nitro N-nitrosoguanidine, or inhibit NAD depletion caused by this agent. Therefore the evidence suggests that CB 1954 is not an inhibitor of poly(ADP ribosylation).