Some time ago, Derek Lowe has blogged about a “Huge But Effective” compound in his fantastic blog “In the Pipeline”. I tried to comment on his post, but somehow my comment has been eaten up during submission. So here I go again (in a little more depth…), this time using my own newly established blog…

Being a natural product scientist, what wondered me most were the comments that followed on Dereks post. In my eyes this compound is not a “big ugly brute” – it is just wonderful! And this is not because natural product scientists in general are crazy, but because I believe in the value of larger compounds, especially if they are peptide-like. There are so many examples of even larger (mostly natural product derived) compounds that are invaluable drugs used in the clinic. “Large” compounds, and even more large and cyclic ones, are a largely unexplored chemical space. For anyone interested in the matter I recommend reading this paper in Nature Reviews Drug Discovery, which very nicely covers the topic and discusses several important “large” compounds in detail (although it sadly is behind the pay wall…).

But from my experience I know that there are still many people who need to be convinced of the fact that “large” does not mean “uninteresting” from a drug development point of view. I work with cyanobacteria, and these microorganisms tend to synthesize large compounds. These compounds are often derived from a polyketide synthase / non-ribosomal peptide synthetase biosynthetic pathway, and the “average cyanobacterial compound” has a molecular weight of about 650 Da. So you can imagine that there are many compounds that are even larger than this. However, we often find stunning bioactivities for these compounds. More on this later.

Most people’s fears revolve around pharmacokinetics. For example, Derek in his post is somewhat surprised that this compound is actually passing cell membranes and doubts that a compound like this would be orally available. Furthermore, he expects short half-lives for this type of compounds. And this is exactly what I hear when it comes to large peptidic compounds from cyanobacteria: “Peptides are not suitable as drugs because they are not stable in the GIT, they are rapidly degraded by proteases / peptidases, they cannot cross cell membranes and are thus not suitable for intracellular targets.” However, all of these (and more) prejudices are dealt with in the Nature Rev Drug Disc paper mentioned above.

When I talk to people about cyanobacterial natural products, I am often confronted with the standpoint that compounds violating the “do not go beyond 500 Da” rule are not suitable as drug substances / leads. I then usually tell three stories about cyanobacterial compounds that clearly show that you can use these compounds in drug discovery and development anyways.

1. Microcystins are known as hepatotoxins produced e.g. by the cyanobacterial genera Microcystis, Oscillatoria, Planktothrix, Nostoc, and Anabaena. They po­tently inhibit the eukaryotic protein phosphatase families PP1 and PP2A1, disrupting intracellular signaling pathways as well as cytoskeleton maintenance. Over 90 natural variants are known, and the average weight of the microcystins is about 1010 Da. The structure of one of the most common microcystins, microcystin LR, is shown below. The WHO has established a limit for microcystins in drinking water (1 µg/L) – now why would the WHO do this if microcystins were not orally available, and stable enough to be toxic after absorption? However, it is true that microcystins – as one could expect – have a poor membrane permeability. Indeed, they need to be actively uptaken by cells to exert toxicity. Three human proteins are able to mediate this uptake, the organic anion transporting polypeptides (OATP) 1B1, 1B3, and 1A2. This “selective” uptake, however, is not a disadvantage. In fact, it can be exploited, and microcystins are currently studied for their potential as leads for anticancer drugs. For more details just have a look at a poster we recently presented at the ICNPR 2012 in New York.
   
2. Cryptophycins are cyclic depsipeptides found in Nostoc sp.. The structure of cryptophycin-1, 668 Da, is shown below. These compounds show remarkable cytotoxic activity in low picomolar (!) concentrations, and represent the most potent suppressors of microtubule dynamics yet described. They block the cell cycle at the G₂/M phase with a 100- to 1000-fold higher activity than paclitaxel and vinblastine, leading to cell death. In addition, they are not effective substrates of the P-glycoprotein, and thus remain active against multidrug-resistant cancer cells. It is not known how they enter the cells, but enter them they do – otherwise they would not find any tubulin to bind to… Eli Lilly and sanofi-aventis both had closer looks at these compounds.
   
3. Dolastatins are linear pentapeptides (750-850 Da; dolastatin 15 shown below) from cyanobacteria. Like the cryptophycins, they bind to tubulin, and display ED₅₀ values in the picomolar range against a number of cancer cell lines. They are highly toxic in vivo (proving that they also find their target in vivo), but this did not hinder scientists to pursue this class of compounds. In fact, brentuximab vedotin is the first approved drug based on a cyanobacterial metabolite. So even linear peptides in this molecular weight range can do it – but that is a longer and somewhat twisted story for another day…