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

N-(Hydroxymethyl) melamines

1. The N-(hydroxymethyl) melamines are analogs of the antitumor agent hexamethylmelamine (HMM) which do not require bioactivation to exert their antitumor effects. 2. Trimelamol (N2,N4,N6-trihydroxymethyl-N2,N4,N6-trimethylmelamine; TM) was developed as a water-soluble antitumor agent for intravenous administration. 3. Phase I and II trials of TM showed promising activity versus platinum-refractory ovarian cancer, but unfortunately further clinical development was halted due to formulation difficulties. 4. Stable analogs of TM were synthesized in an effort to overcome this shortcoming and these were evaluated in a number of in vitro and in vivo studies. 5. While the stable analogs showed good in vitro cytotoxicity in tumor cell lines, only one analog, CB7646 [bis-N-(hydroxymethyl)trimethylmelamine], showed comparable in vivo antitumor activity to that seen for TM. 6. Both TM and CB7646 were curative in human ovarian and breast cancer xenograft models, including the HX110P ovarian cancer xenograft with acquired resistance to carboplatin. 7. As CB7646 possesses favorable formulation characteristics, relating to its superior stability over that for TM, it is currently being developed for phase I clinical trial. 8. The N-(hydroxymethyl) melamines are capable of overcoming many forms of drug resistance, based on data obtained in in vitro and in vivo studies, and thus show promise as agents in the treatment of heavily pretreated, refractive tumors.

Altretamine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic potential in cancer chemotherapy

Altretamine (hexamethylmelamine) is a cytotoxic antineoplastic agent which appears to require metabolic activation. Metabolic intermediates may act as alkylating agents; however, altretamine is not directly cross-resistant with classical alkylating agents. Objective response rates to orally administered altretamine as salvage therapy in patients with advanced ovarian cancer were 0 to 33%, with disease stabilisation in a further 8 to 78% of patients. Response rates appear to be higher in patients who have responded to previous alkylating agent or cisplatin-based therapy. There is some evidence that addition of altretamine to platinum-based combination regimens used for induction therapy of advanced ovarian cancer may improve long term survival, particularly in patients with limited residual disease. Although altretamine displays some activity in small cell lung cancer, it is unlikely to have any clinical role in the management of non-ovarian cancer. Altretamine appears to be relatively well tolerated, with gastrointestinal, neurological and haematological toxicities being the main dose-limiting adverse effects. However, assessment of accurate incidence rates for these effects is complicated by the use of altretamine with cisplatin. On the basis of the emerging body of clinical evidence, altretamine appears to have a limited role in the treatment of persistent or recurrent advanced ovarian cancer, primarily in patients who are potentially platinum sensitive yet intolerant of platinum analogues. Additionally, altretamine may be added to platinum-based regimens for induction therapy of advanced ovarian cancer. At the doses currently recommended, altretamine offers a reasonably well tolerated regimen that can be administered orally and is suitable for use on an outpatient basis.

Stable analogues of the antitumour agent trimelamol retain in vitro cytotoxicity in drug-sensitive and resistant rodent and human cell lines

In spite of clinical activity in heavily-pretreated ovarian cancer, the antitumour s-triazine trimelamol [TM; tris(hydroxymethyl)-tris(methyl)melamine] had to be withdrawn from further clinical studies due to formulation difficulties related to instability. A synthetic programme has produced tris(hydroxymethyl) analogues containing electron-withdrawing groups in place of methyl-triscyanomethyl CB 7669, tristrifluoroethyl CB 7639, CB 7529 and trispropargyl CB 7547, all showing markedly superior stability to TM. Chemosensitivity testing of analogues (MTT assay, continuous exposure) using a panel of rodent and human cell lines showed activity close to that of TM, e.g. for the CH1 human ovarian cancer cell line. IC50 values were TM 23.4 microM, CB 7639 30.5 microM, CB 7529 29.5 microM, CB 7547 28.5 microM and CB 7669 27.3 microM. CB 7669 and CB 7639 required prolonged exposure (> 12 h) in order to exhibit equivalent cytotoxicity to a 2-h exposure to TM. Thus, rather than administration as a single daily dose, the stable analogues may be more suited to prolonged infusion, which was suggested as being a more beneficial regimen in clinical trials with TM. In line with clinical observations indicating the efficacy of TM in platinum-refractory ovarian cancer, we saw no significant cross-resistance to TM or CB 7529 in a range of platinum-sensitive and acquired-resistant cell line pairs or in an alkylating-agent resistant cell line, despite TM's ability to crosslink DNA. Data obtained using cell lines with acquired resistance to TM, CB 7669 and formaldehyde (released in the breakdown of TM) suggest a pivotal role for formaldehyde and a more minor role for alkylating activity in the mechanism of action of the N-(hydroxymethyl)melamines in vitro. Further clinical trials of these compounds are eagerly awaited, and their usefulness as second-line chemotherapy for heavily pretreated ovarian cancer deserves further investigation.

Hexamethylmelamine: pharmacology and mechanism of action

Several conclusions can be drawn from a review of HMM preclinical and clinical pharmacology data. The drug is extensively metabolized by animals and by man. The drug is well absorbed following oral administration to animals, but oral bioavailability is low due to first pass metabolism. Based on limited human data and more complete animal data, absorption of HMM following oral administration may be quite high in man. We do not yet know the oral bioavailability of HMM in patients, but again based primarily on animal studies, oral bioavailability is most likely low and variable due to extensive first pass metabolism. Systemic exposure to HMM and demethylated metabolites following oral administration varies greatly from patient to patient and is sometimes quite low. Most patients are, however, exposed to a substantial fraction of the administered dose when determined by urinary recovery of the total dose (based on parent drug and metabolites or total radioactivity) or by the total plasma AUC of parent drug and all metabolites. Systemic exposure to HMM following intravenous administration is clearly greater and less variable than following oral administration. An unresolved question is whether the highly variable and often low systemic exposure after oral administration compromise antitumor activity when compared to intravenous administration. A key issue is whether or not one accepts the hypothesis that metabolism is a prerequisite for antitumor activity. The metabolic activation studies do not rule out other mechanisms of HMM antitumor activity. Modest activity of HMM was observed after prolonged exposure to cells which did not metabolize the drug. However, most of the accumulated data are consistent with the metabolic activation hypothesis. Certainly HMM has clinical activity when administered by mouth. If metabolism is required, then exposure to the total dose (parent drug and metabolites) could be of significance even when exposure to HMM is low, since every demethylated metabolite must have come ultimately from the initial HMM demethylation. We do not know whether the initial metabolic reaction (occurring in the liver rather than in the tumor) provides sufficient exposure of tumor to reactive species. Specifically, does the variable HMM plasma AUC seen after oral administration lead to variable delivery of potentially reactive species to tumor (by rapid breakdown and/or further metabolism of MPMM before it leaves the gut and/or liver) or are quantities of MPMM delivered to tumor comparable to those delivered following intravenous administration. The issue of rate of MPMM formation compared to rate of breakdown and ultimate delivery to tumor has been noted by Judson and Rutty.(ABSTRACT TRUNCATED AT 400 WORDS)

The bioavailability of three altretamine formulations

The bioavailability of two altretamine preparations was studied in a randomized cross-over design. The two preparations were compared with a third in a parallel design. Dissolution differences between the preparations were observed, which could give rise to differences in bioavailability caused by the extensive first-pass effect of altretamine. The in vivo data showed a trend to differences in bioavailability.

The formation and metabolism of N-hydroxymethyl compounds--IX. N-(acetoxymethyl)-4-chlorobenzamide: an electrophile but not a mutagen in Salmonella typhimurium

The electrophilic properties of 4-chloro-N-(hydroxymethyl)benzamide as a model compound of carbinolamides formed during the metabolic oxidation of N-methylamides were investigated. 4-Chloro-N-(hydroxymethyl)benzamide did not react with nucleophiles such as cyanide or glutathione under physiological conditions. In contrast, N-(acetoxymethyl)-4-chlorobenzamide yielded the cyanomethylamide with KCN and S-(4-chlorobenzamidomethyl)glutathione with glutathione. Under nonaqueous conditions, N-(acetoxymethyl)-4-chlorobenzamide reacted avidly with ethanethiol, with methanol and with diethylamine in the presence of base, whereas 4-chloro-N-(hydroxymethyl)benzamide did not afford products under these conditions. These results show clearly that N-(acetoxymethyl)-4-chlorobenzamide is the precursor of reactive electrophilic methyleneimines. 4-Chloro-N-(hydroxymethyl)benzamide was not biotransformed to such electrophilic species when incubated with mouse hepatic microsomes or a microsomal supernatant with acetyl-CoA or a PAPS generating system. Neither 4-chloro-N-(hydroxymethyl)benzamide nor its acetate ester were mutagenic in the short term bacterial assay using Salmonella typhimurium. Nevertheless, esters of carbinolamides, such as N-(acetoxymethyl)-4-chlorobenzamide, might possess toxic or carcinogenic properties.