Biomedical Engineering Reference
In-Depth Information
respiration inhibitors that are highly toxic to eukaryotic cells. Therefore, if one were screening
in a rodent model for antiparasitic activity, a crude extract containing both the milbemycin and
oligomycin would likely only show the toxicity. On the contrary, if the components were resolved
chromatographically prior to screening the cryptic antiparasitic effect of the milbemycin would also
be observed.
Apart from unmasking activities, there are benei ts that accrue from enhancing the concentration
of minor components present in a complex mixture. This is particularly true if in the preparation
of fractionated screening samples the effort is made to normalize the concentration of the samples.
The benei ts of obtaining the maximum positive responses from these samples, often representing
precious material collected under unique conditions, argue for expending the extra effort required.
In the course of resolving “hits” from natural product screening through bioassay-guided frac-
tionation, as shown in Figure 6.6, it was emphasized that this is an empirical and highly experi-
mental process, each new extract requiring an individual strategy for the isolation of its biologically
active principles. If one has prefractionated the extract prior to initial screening, and the activity
falls into a neat cluster of fractions, then one has valuable information on how to begin the purii ca-
tion process. This information will facilitate the resolution of the hit and lead to greater efi ciency
of the entire isolation and purii cation process.
6.5 OPTIMIZATION OF NATURAL PRODUCT LEADS
Nature has preserved the ability of a given organism to make these fascinating secondary metabo-
lites, although their inherent biological roles remain obscure. As scientists seek to co-opt these
metabolites as medicinal agents, attempts are typically made to enhance their pharmaceutical effec-
tiveness. Such enhancements may be to improve the spectrum of activity against a range of targets,
as in the case of antibiotics where broad-spectrum activity in inhibiting the growth of both gram-
positive and gram-negative bacteria is important. In other cases it may be crucial to enhance the
specii city to a narrower range of targets, such as the ability to selectively inhibit a particular kinase
reaction in a signaling cascade. Furthermore, it may be necessary to improve the drug-like prop-
erties of a natural product lead. Here improvements in solubility, chemical stability in biological
matrices or metabolic stability may be crucial. These and a host of other reasons drive the process
to make structural modii cations of the core natural product, which may be effected by chemical or
biosynthetic means.
6.5.1 S
EMISYNTHESIS
Semisynthesis refers to the process of performing synthetic chemical transformations starting with
a natural product, for the purpose of enhancing the pharmaceutical performance of the natural
product. This approach has been most effectively used with complex microbial products, owing to
the ready availability of the starting material through fermentation of highly productive variants of
the parent organism. The challenge in these experiments is twofold: one, to achieve adequate selec-
tivity in the chemical process and two, the subsequent purii cation of reaction mixtures. A good
example of the successful application of semisynthesis is in the case of the rifamycin antibiotics,
shown in Figure 6.9. Rifamycin B is the originally isolated natural product derived from
Nocardia
mediterranei
. Rifamycins have potent activity against gram-positive bacteria and are of greatest
importance to inhibit the growth of the tuberculosis causing organism
Mycobacterium tuberculosis
.
Rifamycin B was only modestly effective when administered to infected animals and this led to
investigation of derivatives in search of improved potency. Greater potency was achieved through
substitutions on the aromatic portion as exemplii ed by rifamide, rifampicin, and rifabutin. The
latter two compounds continue to be important drugs for the treatment of reemerging epidemics of
tuberculosis.
