Pre-clinical development of the anti-tumour agent CB 7646,BIS N-(hydroxymethyl) trimethylmelamine, a stable analogue of trimelamol

Abstract

The s-triazine hexamethylmelamine (HMM) is perhaps the best known of the group of anti-tumour agents known as the methylmelamines. Despite its introduction into the clinic in a phase I trial some 30 years ago (Wilson and de la Garza, 1965), the role of HMM both in terms of its mechanism of action and as an anti-tumour agent has not been defined. As a single agent in advanced ovarian canccr, metastatic breast cancer, refractory lymphoma and metastatic small-cell lung cancer, HMM has been found to have significant activity comparable to that of classical alkylating agents. It has been employed in combination regimens, where an increased therapeutic effect has been demonstrated with cisplatin, doxorubicin and cyclophosphamide (HCAP) over cisplatin and cyclophosphamide therapy. However, clinical studies have indicated its dose-limiting toxicity to be gastrointestinal, involving nausea, vomiting, anorexia and diarrhoea (I-cgha el aL, 1976).The low aqueous solubility of HMM usually necessitates oral administration (p.0.). However, a formulation involving intralipid and HMM hydrochloride has been used for parenteral infusion (i.v.) in phase I clinical trial (Ames et al., 1990). Trimelamol (tris N-[ hydroxymethyl]trimethylmelamine; TM) was first synthesised in 1977 as a water-soluble derivative of IIMM, representing a third-generation analogue with potential advantages over the parent drug. As metabolism studies have indicated a requirement for HMM to be metabolised to cytotoxic N-( hydroxymrthyl) derivatives, TM, therefore, represents a bioactivated form of hexamethylmelamine. The importance of using such an analogue is highlighted by the ohservation of high levels of N-(hydroxymethyl) derivatives in HMMtreated mice but significantly lower levels in patients. In addition, a clinical study in ovarian cancer patients using HMM described wide interpatient variability in overall drug exposure ( A synthetic analogue programme has been set up in an effort to circumvent the aforementioned problems. A number of stabilised analogues have been made, including some tris N-( hydroxyme t hyl)-subst itu ted de rivat ivcs containing methyl groups replaced by electron-withdrawing substituents (Jarman et al., 1993). In addition, CB 7646, the bis N-(hydroxymethyl) derivative of TM, which possesses some important advantages over the parent compound, has been synthesised (Fig. 1).T M is typically prepared by the reaction of trimethylmelamine in concentrated aqueous formaldehyde, adjusted to pH 9.0. However, we now report that the reaction between 2,4,6-tris(niethylamino)-l,3,5-triazinc and dilute (3% w/v) aqueous formaldehyde in the presence of potassium carbonate afforded a mixture of mono-, his-and tris(hydroxymethy1)trimethyl melamines in which 2,4-his[(hydroxymcthyl)(methyl)-amino]-6-methylamino-1,3,5-triazinc, CB 7646, was the principal component. The concentration of formaldehyde was critical; when the reaction was conducted in 6% w/v formaldehyde under otherwise identical conditions, the product was TM of 97.5% purity.'To whom corr...