Current Pharmaceutical Design - Volume 7, Issue 13, 2001

Vinca alkaloids represent a chemical class of major interest in cancer chemotherapy. The lead compounds vinblastine and vincristine have been employed in clinical practice for more than thirty years and remain widely used to this day. Several hundred derivatives have been synthesised and evaluated for their pharmacological activities, the majority being modified in the vindoline moiety, bearing several reactive centers. These efforts led to the identification of the amido derivative vindesine, registered in Europe in 1980 and now available in several countries. Then novel chemistry permitted the semisynthesis of derivatives modified in the velbenamine or upper part of the molecule, creating a new potential in the Vinca alkaloids medicinal chemistry: as a result, vinorelbine, obtained by C ring contraction of anhydrovinblastine, and is now marketed worlwide. Several strategies aimed at the total synthesis of vinblastine derivatives have been investigated, giving the opportunity to design rationaly certain compounds. Modifications in the D' ring appeared to induce dramatic changes in the tubulin interactions. These observations have been confirmed recently by the identification of unprecedented pharmacological properties exerted by the novel fluorinated Vinca alkaloid, vinflunine. This review will focus more specifically on derivatives which have been modified in the velbenamine part, with the aim of inducing different chemical and pharmacological properties.

The pharmacological profile of vinflunine, a novel bi-fluorinated derivative of vinorelbine is summarised. Detailed comparisons, based on in vitro and in vivo experimental preclinical data, of vinflunine with its parent molecule and the classic Vinca alkaloids, exemplified by vincristine or vinblastine, have revealed certain qualitative and quantitative differences between these first and second generation Vincas. Evidence is gradually accruing indicating that certain more subtle mechanistic differences exist in relation to the precise interactions of these individual molecules with cellular microtubules involving, for example, their suppression of microtubule dynamics. It is tempting, but premature, to suggest that these may be associated with the markedly superior in vivo antitumour activity of vinflunine documented in a series of murine and human xenografted tumour models. The in vivo antivascular effects of vinflunine, identified at doses below those required for optimal antitumour activity, coupled with the demonstrated potential value of vinflunine as a component of combination regimens, together with the finding that resistance to vinflunine was generated far less readily than to vinorelbine, augur well for the ongoing clinical development of this new agent. Finally, it is proposed that as our knowledge of the basic events involved in initiation and completion of mitosis and in defining the precise, yet multifacted, functions of microtubules increases, alternative intracellular targets will be identified. Such targets may prove suitable for pharmacological exploitation and more effective antimitotic antitumour agents will undoubtedly emerge. However, whether these will be third generation Vincas or molecules with quite different structures remains an open question

(250 words): A diverse group of natural biological compounds bind to microtubules and suppress microtubule dynamics. Here we review the mechanism of microtubule assembly and dynamics as well as structural features that are important for nucleotide binding, GTP hydrolysis and stabilization of longitudinal and lateral protofilament contacts. Specific emphasis is placed upon the polar structure of the microtubule, the exposure of the nucleotide hydrolysis site at the + end and the conformational and configurational plasticity of the microtubule lattice. These features have important implications for the mechanism of dynamic instability and the disruptive action of antimitotic drugs. We then discuss the various classes of tubulin binding drugs emphasizing their site and mode of binding as well as the structural and energetic basis for their effects on microtubule assembly and dynamics. A common feature of tubulin-interacting compounds is a linkage to assembly, either the stabilization of a microtubule lattice by compounds like taxol or epothilone A, or the preferential formation of alternate lattice contacts and polymers at microtubule ends by compounds like colchicine, vinca alkaloids and cryptophycin-52. Finally, we explore the likely possibility that these drugs also disrupt the regulation of microtubule dynamics. Future generations of these compounds may be selectively developed to directly target the proteins that regulate mitotic spindle dynamics.

The antimitotic agents of the Taxoid series are important substances as anticancer drugs. The efficacy of paclitaxel (Taxol) and docetaxel (Taxotere) has been well demonstrated in the treatment of breast, ovarian and lung cancers. Although these drugs have brought benefits in cancer chemotherapy, they unfortunately possess some disadvantages due to their inefficacy on certain resistant cancers, and due to their toxic side effects. For these reasons, the synthesis of new taxoids with improved biological activity is still an important research area. This review while covering general aspects of taxoid chemistry focuses on recent developments in this area especially those contributing to the improved availability of antimitotic taxoids. Studies of structure-activity relationships are also described.

Paclitaxel and docetaxel are two key molecules in the treatment of a variety of cancers with major impact in the treatment of breast, lung and ovarian cancers.A number of taxoids have then been synthesized in an effort to improve some of the features of the existing drugs. Although the literature is still scant of preclinical data due to the highly competitive field, several compounds are already in clinical trials. Most of these will be reviewed and have, either improved water solubility or reduced cross-resistance with marketed taxoids or reduced interaction with P-glycoprotein.In addition, the reduced recognition of several compounds by multi-drug-resistance-related transport systems has yielded some orally bioavailable compounds with marked in vivo antitumor activity.It is likely that these additional properties should lead to an expanded spectrum of clinical activity compared to that of clinically available taxoids.

The antitumor cryptophycins are synthetic derivatives of the desipeptide cryptophycins isolated from the cyanobacterium Nostoc sp. Cryptophycin 52 that is currently in clinical trial in solid tumors, is prepared by total synthesis of four key fragments that are coupled to form the final product. The mechanism of anticancer activity of the cryptophycins has been associated with their destabilization of microtubules and with their induction of bcl-2 phosphorylation leading to apoptosis. The cryptophycins maintain activity against ovarian and breast carcinoma cells that overexpress the multidrug resistance efflux pump P-glycoprotein. Cryptophycin 52 has demonstrated a broad range of antitumor activity against both murine and human tumors. In the human MX-1 breast carcinoma xenograft cryptophycin 55 produced greater-than- additive tumor response in combination with 5-fluorouracil. In human non-small cell lung carcinoma and human small cell carcinoma xenografts, administration of the cryptophycins along with gemcitabine, cisplatin or carboplatin resulted in antitumor activity greater than either agent alone. The cryptophycins appear to be additive with fractionated radiation therapy in the human H460 non-small cell lung carcinoma. In the human HCT116 colon carcinoma, the cryptophycins resulted in a greater than additive tumor response when administered sequentially with 5-fluorouracil or irinotecan. Treatment of animals bearing intraperitoneal human OVCAR-2 ovarian carcinoma with cryptophycin 52 resulted in survival times that were greater than those achieved with docetaxel or paclitaxel. Cryptophycin 52 is currently in early clinical testing.

Taxol is currently one of the most effective anticancer agents available. However, limitations due to multidrug-resistance (MDR) susceptibility and lack of aqueous solubility render it less than an ideal drug. These limitations, coupled with taxols unique mechanism of tumor inhibition, involving the stabilization of microtubule assembly, have spurred the search for more effective chemotherapeutic agents. This review will discuss the chemistry and biology of some of the most promising new molecules with taxol-like activity. The extended family of microtubule-stabilizing agents now includes the epothilones, eleutherobins, discodermolide, laulimalide and WS9885B. The epothilones have emerged as one of the most exciting new candidates for detailed structure-activity-related studies. A review of our efforts in the synthetic and biological aspects of this research is presented, as are the latest developments reported from other laboratories in academia and the pharmaceutical industry. The synthesis and structure-activity studies of eleutherobins, as well as recent progress with discodermolide, laulimalide and WS9885B are also reviewed. An abundance of exciting advances in chemistry and biology have emerged from these studies, and it is hoped that it will ultimately result in the development of new and more effective chemotherapeutic agents in the fight against cancer.

Recently identified novel agents that disrupt tubulin polymerization include synthetic spiroketal pyrans (SPIKET) targeting the spongistatin binding site of beta tubulin. These agents exhibit anticancer activity by disrupting normal mitotic spindle assembly and cell division as well as inducing apoptosis. At nanomolar concentrations, the SPIKET compound SPIKET-P caused tubulin depolymerization in cell-free turbidity assays and exhibited potent cytotoxic activity against cancer cells as evidenced by destruction of microtubule organization, and prevention of mitotic spindle formation in human breast cancer cells. SPIKET compounds represent a new class of tubulin targeting agents that show promise as anti-cancer drugs.

Besides the many recognised compounds described and detailed in the present issue including the Vinca alkaloids, the taxanes, certain cryptophycines, epothilones and eleutherobines, several new products interacting with tubulin are identified regularly in the literature. These products may have been isolated from natural sources (plants, marine organisms, bacteria), but also more recently combinatorial, or at least automatised chemistry, has provided new families of small molecules, which on occasions have been found by High Throughput Screening directed against tubulin as a specific target. A recent review has listed more than one hundred of such derivatives [1]. Certain of these are in an advanced stage of pharmaceutical development, as reviewed by Li et al. [2] and von Angerer [3]. From a mechanistic point of view, these newer products may be classified into one of three main families, although exceptions to this rule are now also being reported on. microtubule stabilising compounds, Vinca alkaloid site interacting agents, colchicine site binders.This brief final contribution to this volume will focus only on the newly identified products for which active pharmaceutical development has been reported on during the last three years.