Both of Arthur Barclays samples of Taxus brevifolia were
sent to the Wisconsin Alumni Research Foundation for extraction. One of
the samples, PR-4960, the bark, showed cytotoxicity against KB. Though
the in vivo tests on L1210 leukemia yielded mixed results, further
KB tests confirmed anticancer activity. Accordingly, the crude plant extract
was designated NSC670549 to conform to CCNSC protocols. And Jonathan Hartwell
of the Center instructed his counterpart at the USDA, Robert Perdue, to
take steps to collect larger quantities of the bark of T. brevifolia
for further evaluation.
On September 8, 1964 Barclay returned to the spot where he had originally
collected PR-4960 and bagged thirty pounds of bark from T. brevifolia,
which was shipped to Monroe Wall at the Research Triangle Institute after
other laboratories turned it down. These laboratories were reluctant because
plants that exhibited high cytotoxicity turned out to be poor drug candidates
since they are toxic to everything, killing healthy cells as well as cancerous
ones. But Wall was interested because of his experience with Camptotheca
acuminata, which was highly cytotoxic but from which his group had
isolated a compound that showed great promise.1
When it received Barclays shipment of bark, RTI began a bioassay-directed
fractionation to isolate the pure compound in T. brevifolia responsible
for its anticancer activity. In September 1966 the researchers isolated
from one of the fractions a crystalline substance they designated K172
(after NCI practice in which pure compounds were identified with a K
prefix). Since it was common practice for a laboratory to name compounds
extracted from material assigned to it, Wall decided to name the substance
taxol. He came up with the name for two reasons. First, while
the RTI scientists did not yet know the compounds complete structure,
they were sure it contained some hydroxyl groups, signifying it was an
alcohol. And second, it was common to name a molecule after the genus
of the plant from which it came. Accordingly, the molecule became known
as Taxol® tax for Taxus and ol
for alcohol. Besides, the name had a nice ring to it.2
Initially, Mansukh Wani reported some success in characterizing the isolated
compound. But by early 1967 Wani had to admit failure in his attempts
to characterize the active compound, unable even to determine its molecular
weight. This initial work on the Pacific yew coincided with the isolation
of camptothecin, which was looking very hot and on which NCI
was ready to begin clinical trials.3 Moreover, RTI had many
other plant extracts to evaluate.
At that point Wall told Wani to forget about Taxus brevifolia.
But Wani persisted. He refused to give up on the yew completely, continuing
to work on it on a low priority basis. Even this proved difficult
because, as Wall wrote thirty years later, the basic problem was
that NCI had at that time (1966-1973) a rather rigid protocol which had
to be met in order to obtain larger quantities of a natural product, and
for carrying out expensive antitumor assays, toxicology assays, etc.4
The need for more and more bark was compounded by the fact that the yield
of Taxol® from collected bark was very low. This slowed the process
to determine the molecules structure. To definitively learn the
structure of Taxol®, Wall and Wani used mass spectrometry, X-ray crystallography,
and NMR spectroscopy, a technique then in its infancy. These various tools
produced the molecular structure with a formula of C47H51NO14.
Wall and Wani also discovered that Taxol® was two molecules in one
a large molecule connected to a small tail or side chain. In 1971
Wall, Wani, and co-workers published the results of their work on Taxol®
in the Journal of the American Chemical Society as a communication
to the editor in which they stressed the antileukemic and antitumor activity
of the newly isolated and elucidated molecule.5 Interestingly,
the structure of Taxol® is so complex that Wall and Wani almost went
to press with the side chain of the molecule placed in the wrong position.6
Wall and Wani at this point passed Taxol® to the NCI for further study
of its anticancer properties in the hope that advanced animal studies
and toxicological tests would eventually lead to human clinical trials.
But NCI administrators showed great reluctance to pursue Taxol® for
a number of reasons. Isolation and extraction of Taxol® from T.
brevifolia was arduous and the tree, not all that prevalent, produced
only small amounts of the compound. Each tree yielded only about two kilograms
of bark and it took in the early days of testing about 12 kilograms of
dried bark to obtain one-half gram of Taxol®. Moreover, bark is a
finite resource; a tree stripped of its bark dies.
Because of the supply problem Taxol® literally stayed on the shelf
at RTI for the next few years. But then several things happened to bring
renewed interest in Taxol®. Matthew Suffness joined NCI, conducting
a review of the natural products program and encouraging tests that showed
Taxol® very active against B16 melanoma, a newly adopted tumor model.
Almost at the same time, NCI introduced other new tumor models, called
xenografts, in which human tumors are planted in what are called nude
mice. These specially bred mice are called nude because they are hairless.
More importantly, these mice lacked a functioning immune system, which
meant their bodies would not reject transplanted human tumors. In November
1978 Taxol® showed the ability to cause considerable regression in
a mammary tumor xenograft. And finally, interest in Taxol® increased
greatly in 1979 when Susan Horwitz demonstrated that Taxols®
mechanism of action was unique.
1 Interview with Dr. Mansukh Wani conducted by Judah Ginsberg
on January 13, 2003.
2 Research Triangle Institute, Progress Report No. 21, June
26, 1967; Wall, Monroe and Mansukh Wani, Camptothecin and Taxol:
Discovery to Clinic, Cancer Research 55, 1995, p.
3 Inteview with Dr.Wani.
4 Monroe Wall to Jordan Goodman, October 21, 1996.
5 Wani, M. C., H. L. Taylor, Monroe Wall, P. Coggon, A. T.
McPhail, 1971, Plant Antitumor Agents. VI. The Isolation and Structure
of Taxol, a Novel Antileukemic and Antitumor Agent from Taxus brevifolia,
Journal of the American Chemical Society, 93: 2325-2327.
6 Interview with Dr. Wani.