Discovering camptothecin

Discovering camptothecin^™

In 1966 Monroe Wall wrote of his work at the Eastern Regional Research Laboratory: “The joint effort of chemists and botanists proved to be a good model for future programs. Indeed, it firmly established the fact that the close cooperation between chemists and botanists was required for a successful natural products program.”¹

Monroe Wall was born in Newark, New Jersey, in 1916 and earned his undergraduate and graduate degrees from Rutgers University. In 1941 he joined the USDA’s ERRL in Philadelphia where he worked on potential agricultural alternatives for products, such as rubber, critical to the war effort. After the war, he became involved in plant screening, and in 1957 a chance visit from Jonathan Hartwell of the Cancer Chemotherapy National Service Center changed the course of Wall’s career.

It was Hartwell who convinced Wall to send NCI one thousand ethanolic plant extracts for antitumor activity testing. A year later Wall learned that one of the plants demonstrated potent activity: Camptotheca acuminata. But the USDA was not interested in anticancer drug research, so Wall’s ambition to identify the active component in C. acuminata had to be put on hold until July 1960 when the Research Triangle Institute recruited him. According to Mansukh Wani, an organic chemist born in Bombay, India, who Wall lured to RTI in 1962, the Institute was “nothing but four ‘walls.’ It was not until the fifth ‘Wall’ arrived that the chemistry programs, in the form of the Natural Products Laboratory, started moving.”²

Wani describes Wall as “a go-getter” and “a very dedicated scientist.” About Wall, Wani adds, “persistence was his greatest virtue, but patience was not.” Wall and Wani collaborated for forty years, a collaboration that began with research into the nature of the compounds responsible for the antitumor activity in C. acuminata.

By 1963 RTI had an approximately 20 kg sample of the wood and bark of C. acuminata available, and Wall and Wani and colleagues at RTI began what they describe as ‘bioactivity-directed fractionation.’ In this the crude plant extract is purified in an iterative manner. At each stage the ‘fractions’ are tested for bioactivity. Those fractions showing the most potent activity are carried on to the next stage of purification. The process is repeated many times until the compound(s) responsible for the bioactivity observed with the crude extract are isolated. In the 1960s this was a difficult and slow process requiring great skill and intuition on the part of the researchers using equipment that can now be found only in museums.

By the time Wall and Wani began fractionating their samples it was known that C. acuminata was very active in the L1210 mouse leukemia assay, which was unusual since most plants did not exhibit such activity. It was this activity that had aroused the interest of NCI. All of the fractions of the extract were analyzed both by the in vivo L1210 mouse life prolongation assay and by the KB in vitro cytotoxicity assay. The pure compound isolated as a result of the fractionation was given the name camptothecin and it was shown to be the agent that was not only very active against L1210 leukemia but also against P388 leukemia cells.

RTI scientists Keith Palmer and Harold Taylor did the original isolation of camptothecin, and Ed Cook worked on determining its structure. At that point, Wall asked Wani, who he described as having “the knack to work on small amounts of material,” to prepare a camptothecin derivative for single X-ray crystallography. The derivative was sent to Andrew McPhail and George Sim at the University of Illinois who reported back within a few weeks the tentative structure of camptothecin, which is unusual although it is related to the indole alkaloids. The research on the isolation and structure of camptothecin was published in 1966 in the Journal of the American Chemical Society, the first paper Wall, Wani, and colleagues published on a natural product with anticancer potential. Wani describes the isolation of camptothecin as “the most exciting scientific event in my life.”³

Because camptothecin showed such promising antitumor activity, the NCI decided to proceed with clinical trials. But camptothecin is not soluble in water (which makes delivery of a potential drug difficult) so the trials were conducted with a water-soluble sodium salt that could be formulated for intravenous delivery. In the trials, some patients with gastrointestinal tumors responded to treatment for a short time.

Despite some encouraging successes, the use of camptothecin as an anticancer agent languished for almost fifteen years until its unique mode of action for killing tumor cells was determined. Camptothecin traps an important cellular enzyme, topoisomerase I, in complexes with DNA. This prevents cancer cell DNA replication and results in the death of the cancer cell. This unique mode of action rekindled interest in developing analogs of camptothecin that were both water soluble and retained anticancer activity. In the mid-1990s, two camptothecin analogs, topotecan and irinotecan, received FDA approval for use against ovarian, lung, breast, and colon cancers.

Wall and Wani worked with many plant samples in the 1960s in addition to C. acuminata. One of these samples came from the bark of Taxus brevifolia, a tree that was to have a much more convoluted history in the annals of drug development than C. acuminata, but which would eventually yield one of the most potent chemotherapy agents yet developed.

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¹ Wall, Monroe and Mansukh Wani, “Camptothecin: Discovery to Clinic,” Annals of the New York Academy of Sciences, Volume 803, December 13, 1996, p. 1.

² Interview with Dr. Mansukh Wani conducted by Judah Ginsberg on January 13, 2003.

³ Ibid. Wall, M. E., M. C. Wani, C. E. Cook, K. H. Palmer, A. T. McPhail and G. A. Sim, 1966, “Plant antitumor agents. 1. The isolation and structure of camptothecin, a novel alkaloidal leukemia and tumor inhibitor from Camptotheca acuminata.” Journal of the American Chemical Society. 88: 3888-3890.

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