Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
1999-05-04
2001-04-17
Kifle, Bruck (Department: 1611)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C546S048000
Reexamination Certificate
active
06218540
ABSTRACT:
BACKGROUND OF THE INVENTION
(20S)-(+)-Camptothecin, a pyrrolo [3,4-b] quinoline alkaloid, is the active principle of the Chinese tree, Camptotheca acuminata Decne, extracts of which have been used for centuries in traditional Chinese medicine to treat cancer. The alkaloid is mostly found in the bark of the plant, a fast growing deciduous tree native only to China and Tibet, where it is known as xi shu (“happy tree”).
Camptotheca belongs to the family Nyssaceae (tupelo family) and may reach a height of about 25 m. It was first introduced to the United States in 1911 as an ornamental plant and on several occasions subsequently. More recently,
C. acuminata
plantations were established in southern Louisiana in an effort to provide raw plant materials for the production of camptothecin. The alkaloid may also be obtained from trees of the species Nothapodytes, which are native to the Indian subcontinent.
1.1 Scientific Interest in CPT
CPT was first isolated by Wall et al. in 1966 during an antitumor screening program, and immediately it generated a great deal of interest, both in the clinical and the chemical arenas, as a new agent for the treatment of human malignancy. The compound itself is highly insoluble in aqueous media, but the sodium salt, obtained by alkaline hydrolysis of the lactone ring, displayed good water solubiliy and was easily formulated for intravenous administration. As a consequence, phase I clinical trials were launched in the early 1970's in patients with an advanced form of gastrointestinal cancer. In retrospect, those protocols were poorly designed (vide infra), but in any event they led to the erroneous conclusion that CPT was only marginally effective. The drug did produce partial remissions in some patients, but the trials were eventually halted because of severe toxic side effects.
1.2 Mechanism of Action: Inhibition of Topoisomerase I
Details of the mechanism of action of CPT remained a mystery until 1985, when Liu et al. discovered that the substance is a specific inhibitor of topoisomerase I (Topo I), an essential enzyme for DNA replication and transcription. Topo I is a monomeric protein with a MW of ca.100 kD that relaxes (i.e., unwinds) torsionally strained (supercoiled) DNA ahead of active transcription and translation sites. The enzyme initially binds noncovalently to double-stranded DNA, but then it creates a transient break in one strand and concomitantly becomes covalently bound to the 3′-phosphoryl end of the nicked nucleic acid strand. The unbroken DNA strand is allowed to unwind once and to pass through the break site, before topo I religates the cleaved DNA and reestablishes the double-stranded configuration These events constitute an obligatory stage of DNA replication and transcription, as the chromosomes must be unwound in order for the cell to express genetic information or to divide. The covalent complex between topo I and a single DNA strand, also termed the “cleavable complex”, is in rapid kinetic equilibrium with the noncovalently bound complex (the “noncleaveable complex”). Camptothecin is believed to express its activity by reversibly binding to the cleavable complex and stabilizing it, thereby inhibiting religation of the nicked DNA strand. As a consequence, the advancing DNA polymerase operating in the replicating fork soon “collides” with the stabilized cleavable complex and creates an irreparable double-strand break, which is fatal to the cell.
Cancer cells are more vulnerable to topoisomerase inhibition than normal cells, because they grow and reproduce at a much faster rate. Even more significantly, elevated concentrations of topo I have been found in many different tumor cells. In 1994, CPT was also found to inhibit both acute and chronic HIV-I infections. These observations have stimulated renewed interest in camptothecin, and as a result, much research in its pharmacology, medicinal chemistry and total synthesis is currently underway. Camptothecin itself has been used in China to treat leukemias and carcinomas of the stomach and liver, but it is not is not approved by FDA for use in the US. Considerably more promising are a number of semisynthetic derivatives currently being tested in phase I and phase II clinical trials. Examples include Gl 147211C, irinotecan (CPT-11), 9-aminocamptothecin (9-AC), 9-amino-10,11-methylenedioxy camptothecin (9-AC-10,11-MD) and topotecan. The latter substance was approved by FDA in May 1996. Positive results have recently been reported in the treatment of diverse neoplasms, such as colon and breast cancers, malignant melanoma, small-cell lung cancer, ovary, leukemia and non-Hodgkin's lymphoma. Toxicity remnains a problem, major side effects including severe diarrhea, nausea, leukopenia, and possibly bone marrow depression.
1.3 Relationship Between CPT Structure and Activity
The relationship between the structure of CPT and in vitro and in vivo activity has been reported in detail, hence this section will summarize only major findings.
1. An intact E ring (lactone moiety) is necessary for activity.
2. The (20S) configuration is an absolute requirement, (20R)-CPT or analogs being inactive in vitro and in vivo.
3. With the exception of SN-38, only ring A derivatives increase antitumor activity. Substitutions at C-9 or C-10 generally enhance activity.
4. The pentacyclic structure of CPT is an absolute requirement. Tetracyclic analogs or bicyclic and tricyclic analogs are inactive.
5. Disubstitution in general leads to CPT analogs with reduced or no activity, the exception being the 10,11-methylenedioxy moiety, which greatly increases activity. Substitution in the 11- and 12-positions gave CPT analogs which exhibited reduced activity or total inactivation.
6. The a-hydroxy lactone moiety in ring E is required.
7. Substitution of nitrogen for OH or lactone oxygen in ring E leads to inactivation.
8. The pyridone ring D is required. Replacement of the pyridone ring by an aromatic ring results in inactivation.
9. The C-20 ethyl substituent is required, although there is some flexibility; for example, substitution of a 20-allyl group resulted in good activity, whereas substitution of a methyl group led to inactivation.
1.4 Biosynthetic Pathway
In 1967 Wenkert et al. hypothesized that CPT is derived biosynthetically from tryptophan and secologanine. Winterfeld later expanded on this idea based on his own finding that indole derivatives underwent facile auto-oxidation to quinolones in vitro.
The proposed biosynthetic pathway of CPT was later verified and further elaborated by Hutchinson via plant feeding experiments with regiospecifically labeled precursors. It seems clear that strictosamide is a key precursor. Transformation of strictosamide into CPT is considered to be possible through ring BC oxidation followed by recyclization, ring D oxidation, removal of the C-21 glucose moiety and ring E oxidation.
1.5 Previous Syntheses
Further clinical evaluation of CPT, and congeners is intimately dependent on the availability of an efficient total synthesis. The natural product is exceedingly rare and costly. It may be obtained virtually exclusively from the People's Republic of China through unreliable suppliers authorized by the Chinese Government. Large orders (100 g) are priced at ca. $ 35-40/g. The latest (1996-97) catalog from the Aldrich Chemical Co. lists CPT for $ 127.50/g: nine times the current price of pure gold ($ 395/oz=$ 14.10/g). In recent times it has become apparent that certain derivatives of CPT possess more desirable pharmacological properties. The preparation of these substances by semisynthesis is often lengthy and inefficient, because the natural product does not lend itself readily to derivatization, and because its insolubility in most organic solvents creates a host of technical problems. In this light, total synthesis appears to be an entirely acceptable alternative, if not a much better approach. Yet the deceptively simple structure of camptothecin is endowed with robust defenses against a synthetic attack, not the least of which is
Ciufolini Marco
Roschangar Frank
BioNumerik Pharmaceuticals, Inc.
Dodd Thomas J.
Kifle Bruck
LandOfFree
Process for preparing camptothecin does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for preparing camptothecin, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for preparing camptothecin will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2490419