Science: Mitotic Poison, Vincristine

 

Madagascar periwinkle (Catharanthus roseus, Apocynaceae)

Today, we are going to examine another mitotic poison called vincristine, which is a highly valuable anti-cancer drug. Vincristine acts like colchicine and it is strongly cytotoxic, but unlike colchicine, vincristine has a relatively wider therapeutic window. This is a plant toxin (alkaloid) that has saved many lives, and we owe its discovery partly to accident!

 

Figure 1: Chemical structure of vincristine and vinblastine. 

Vincristine and its congener vinblastine are two major cytotoxic alkaloids that were isolated from the Madagascar periwinkle plant (Catharanthus roseus, Apocynaceae). They are classified as alkaloids of the indole skeleton, more specifically bis-indole terpene alkaloids. The prefix 'bis' means that vincristine and vinblastine are dimeric alkaloids that comprise of two individual indole alkaloid unit, which I have highlighted in red and blue, respectively, in Figure 1. Take note of the indole nitrogen atom and don't confuse it with the alicyclic tertiary amine. Indole is aromatic and its nitrogen atom is not basic, it is the two tertiary amine nitrogen atoms that confer vincristine and vinblastine alkaline property. Vincristine only differs from vinblastine at the substituent of one of its indole nitrogen atom (blue), the former having an N-formyl group instead of a methyl group in the latter.  As a whole, the two only differs in a HHO mass unit. There are nine stereocenters in the 3D structure of vincristine and vinblastine, and their indole rings are basically planar. I have labelled the stereochemistry of both alkaloids in Figure 1, and I'll leave you to comprehend the full 3D structure of vincristine in Figure 2. 

Figure 2: 3D structure of vincristine.

Figure 3: Simplified biosynthesis pathway of vincristine and vinblastine.

Remember I mentioned the term monoterpene indole alkaloid? It is a large sub-type of indole alkaloids that occur predominantly in the plant order Gentianales, which include the dogbane (Apocynaceae), coffee (Rubiaceae) and Logania (Loganiaceae) families. Prominent plant alkaloids like strychnine, ibogaine, mitragynine and yohimbine all fall into this category. We can make sense of monoterpene indole alkaloids  by examining the biosynthesis pathway of vincristine (Figure 2). Firstly, a precursor amino acid tryptamine, which yields an indole skeleton condenses with a monoterpene glycoside called secologanin via Pictet-Spengler's reaction. Similar to a diterpene, monoterpene originates from the acetate-mevalonate pathway and it always contain 10 carbon atoms. Thus, an upstream precursor called strictosidine is produced, and it is the origin of all monoterpene indole alkaloids (thousands of them). Subsequently, strictosidine loses its glycosidic (sugar) group, and is transformed via multiple intermediates into two monoterpene indole alkaloids called catharanthine and tabersonine, respectively. Note that a particular ring-closure step called Diels-Alder reaction results in the formation of 5 rings (labelled A–E) in both catharanthine and tabersonine. Tabersonine is then converted into a more oxygenated monoterpene indole alkaloid called vindoline, which is condensed with catharanthine (ring A of vindoline to ring C of catharanthine) to yield vincristine and vinblastine. To a lay audience, I admit that this is probably the most challenging biosynthesis pathway we have encountered! You are certainly forgiven if you can't comprehend the exact reaction mechanisms, but I hope you get a vague understanding of how vincristine is a bis-indole monoterpene alkaloid. 

 

Relax, I promise we won't have anymore chemistry in this article.

Vincristine and vinblastine were first discovered by chemists working for the Eli Lilly pharmaceutical company in 1950s. Back then, scientists were actually screening for compounds with anti-diabetic activity from the Madagascar periwinkle plant (Catharanthus roseus syn. Vinca roseus) because it is used traditionally to treat diabetes. This resulted in the isolated and characterisation of more than 150 indole alkaloids, but not a single one produced desirable anti-diabetic effect. Instead, the scientists observed that many of their test animals died of infection due to a lack of white blood cells (leukopenia). This selective cytotoxic effect was most prominent in two bis-indole alkaloids, which were later named vincristine and vinblastine (vin for Vinca). The two alkaloids were later developed into anticancer drugs and they have proved to be extremely invaluable even today. Vincristine is primarily used nowadays to treat childhood leukaemia, as well as certain cancers of the breast, lung and reproductive tract. Vinblastine is somewhat less potent than vincristine, and it's used to  treat Hodgkin's lymphoma, a cancerous disease that affects the liver and spleen. Both alkaloids have to be injected (infusion) to be effective, and they are often used in conjunction with other anticancer drugs to reduce cancer chemotherapy resistance. Just like any chemotherapy drugs, vincristine and vinblastine are highly toxic in overdose, and their use are often associated with side effects such as hair fall and peripheral nerve damage. Humans have died due to medication error when vincristine is injected into the spine (intrathecal) instead of the vein (intraveneous)! For some reasons, vincristine drug delivery is particularly prone to human errors, and there are many documented cases of intrathecal vincristine accidents, the results were invariably fatal. Nonetheless, vincristine remains a most valuable anticancer drug. In fact, the isolation of vincristine from Catharanthus roseus is tedious and costly, because it only accounts for 0.0002% yield (one of the lowest yielding plant alkaloid). More than 500 kg of periwinkle is required to produce just one gram of pure vincristine!  Thus, chemists are always finding more efficient ways to obtain vincristine, even though the periwinkle is already cultivated on an industrial scale. Lucky for us, Catharanthus roseus produces vinblastine in a higher yield, and we can chemically convert vinblastine into vincristine in the lab. We call it semi-synthesis, and it is now possible to semi-synthesise vincristine from its individual monomers catharanthine and vindoline in the lab.

Figure 4: Cytotoxic mechanism of action of vincristine.

Vincristine acts similarly to colchicine and it inhibits the formation of microtuble. The only difference is that vincristine binds to alpha-tubulin instead of the interface between alpha and beta-tubulin heterodimer. When the vincristine bound tubulin heterodimers are unable to replenish the intrinsically unstable microtubule, the microtubule breaks down and is unable to regenerate. The dividing cells then commit suicide.

That's it for today and here's your homework. There are many other cytotoxic/ mitotic poisons in the plant kingdom. What do you think will happen if we have a toxin that stabilises the intrinsically unstable microtuble, and prevents it from breaking down when it has to? What other plant-based anticancer drug is so valuable but has an even lower yield than vincristine? What's more it originates from a deadly poisonous plant that can be hundreds of years old? What happens if we chop it down for the anticancer drug, what's left for us in the future?