Simple and Efficient Conversion of Colchicine into Deacetylcolchicine

Colchitaxel, a coupled compound made from microtubule inhibitors colchicine and paclitaxel

BACKGROUND

Tumor promoters enhance tumor yield in experimental animals without directly affecting the DNA of the cell. Promoters may play a role in the development of cancer, as humans are exposed to them in the environment. In work based on computer-assisted microscopy and sophisticated classification methods, we showed that cells could be classified by reference to a database of known normal and cancerous cell phenotypes. Promoters caused loss of properties specific to normal cells and gain of properties of cancer cells. Other compounds, including colchicine, had a similar effect. Colchicine given together with paclitaxel, however, caused cells to adopt properties of normal cells. This provided a rationale for tests of microtubule inhibitor combinations in cancer patients. The combination of a depolymerizing and a stabilizing agent is a superior anti-tumor treatment. The biological basis of the effect is not understood.

RESULTS

A single compound containing both colchicine and paclitaxel structures was synthesized. Colchicine is an alkaloid with a trimethoxyphenyl ring (ring A), a ring with an acetamide linkage (ring B), and a tropolone ring (ring C). Although rings A and C are important for tubulin-binding activity, the acetamide linkage on ring B could be replaced by an amide containing a glutamate linker. Alteration of the C-7 site on paclitaxel similarly had little or no inhibitory effect on its biological activity. The linker was attached to this position. The coupled compound, colchitaxel (1), had some of the same effects on microtubules as the combination of starting compounds. It also caused shortening and fragmentation of the + end protein cap.

CONCLUSION

Since microtubule inhibitor combinations give results unlike those obtained with either inhibitor alone, it is important to determine how such combinations affect cell shape and growth. Colchitaxel shows a subset of the effects of the inhibitor combination. Thus, it may be able to bind the relevant cellular target of the combination. It will be useful to determine the basis of the shape reversal effect and possibly, the reasons for therapeutic efficacy of microtubule inhibitor combinations.

Synthesis and Characterization of a Cobalamin−Colchicine Conjugate as a Novel Tumor-Targeted Cytotoxin

Colchicine was derivatized at C7 with p-alkoxyacetophenone and conjugated to cobalamin (vitamin B(12)) through an acid-labile hydrazone linker. The cobalamin moiety leads to preferential uptake of the cobalamin-colchicine prodrug by cancer cells, whereupon the hydrazone linker undergoes hydrolysis in the lysosome to unmask colchicine, which acts as a potent cytotoxin by stabilizing microtubules and causing cell death. The bioconjugate is stable in cell culture media and at neutral pH but undergoes hydrolysis with a half-life of 138 min at pH 4.5. The colchicine-cobalamin bioconjugate exhibits nanomolar LC(50) values against breast, brain, and melanoma cancer cell lines in culture. Attachment of colchicine to cobalamin is expected to increase the therapeutic index of the drug by limiting the side effects caused by the current nonselective administration of tubulin-targeted chemotherapeutic drugs.

The interaction between lipid derivatives of colchicine and tubulin: consequences of the interaction of the alkaloid with lipid membranes

Colchicine is a potent antimitotic poison which is well known to prevent microtubule assembly by binding tubulin very tightly. Colchicine also possesses anti-inflammatory properties which are not well understood yet. Here we show that colchicine tightly interacts with lipid layers. The physical and biological properties of three different lipid derivatives of colchicine are investigated parallel to those of membrane lipids in the presence of colchicine. Upon insertion in the fatty alkyl chains, colchicine rigidifies the lipid monolayers in a fluid phase and fluidifies rigid monolayers. Similarly X-ray diffraction data show that lecithin-water phases are destabilized by colchicine. In addition, an unexpectedly drastic enhancement of the photoisomerization rate of colchicine into lumicolchicine in the lipid environment is observed and further supports insertion of the alkaloid in membranes. Finally the interaction of colchicine with lipids makes the drug inaccessible to tubulin. The possible in vivo significance of these results is discussed.