WARNING: Science Content
As with all my posts, I try to include some thought provoking science, as well as interesting water cooler talk. Good luck.
Liam’s Rounds 1 and 2
The first chemotherapy drugs in Liam’s treatment were topotecan and cyclophosphamide. He received two five-day treatments with these drugs. The first was during Aug 2014 and the second was in Sept. 2014 (separated by four weeks).
The drugs are given together for a combined treatment that offers a synergistic effect. In his case, they proved to be very effective. His cancer could not be found in a PET scan after just one round. (This doesn’t mean that his cancer wasn’t present, just that the much of the solid mass tumors were no longer taking up glucose … in other words – dead or dying.)
Even though chemotherapy drugs are often given in pairs to give a compounded blow to the cancer, in these blog posts I will explain the mechanism of each drug individually so that it can be digested by the audience.
Topotecan is a topoisomerase I inhibitor which is derived from a plant called the Asian “Happy Tree” (Camptotheca acuminata). Topoisomerase I, discussed in a previous blog post, is an enzyme that relieves torsional strain ahead of the advancing DNA replication fork. Without this torsional strain relief, the DNA is too strained to be able to separate into two strands.
A chemical mechanism for how topotecan inhibits topoisomerase I is proposed in the Proceedings of the National Academy of Sciences.  The authors suggest that as the one side of the DNA is pulled apart by topoisomerase I, the topotecan molecule wedges (intercalates) between the +1 and −1 bases of the duplex DNA, and is further stabilized by six different protein contacts while the DNA is open. (Think of jamming a zipper, and then sewing the thing that is lodged in the zipper in place.)The intercalation of topotecan causes a shift of the downstream bases by ~3.6 Angstroms (defining the displacement that this monkey wrench causes in the system). Figures 2 and 3 show ribbon diagrams of both the regular process and the process that is poisoned by topotecan.
An article in Nature  provides evidence that topisomerase I is inhibited (poisoned) more readily when the forming supercoil has a positive coiling direction. Topotecan has the effect of being a monkey wrench in the topoisomerase I process.
In slightly easier to understand terms, the topotecan is just the perfect shape with just the right bonding attachments to act like something getting stuck in a zipper. Eventually, with enough vibrations and thrashing about, the topotecan will get unstuck, but cancer is impatient with the process and typically triggers the apoptosis alarm (cell death trigger) before it can work the topotecan out of the zipper.
- http://chemocare.com/chemotherapy/drug-info/Topotecan.aspx ↩
- PNAS, 99, 24, pp. 15387- 15392 (26 Nov. 2002), “The mechanism of topoisomerase I poisoning by a camptothecin analog” ↩
- Anyone may, without requesting permission, use original figures or tables published in PNAS for noncommercial and educational use (i.e., in a review article, in a book that is not for sale) provided that the original source and the applicable copyright notice are cited. http://www.pnas.org/site/aboutpnas/rightperm.xhtml ↩
- See note 3 ↩
- Nature 448, pp. 213-217 (12 July 2007), “Antitumour drugs impede DNA uncoiling by topoisomerase I” ↩