Camptothecin and taxol: historic achievements in natural products research

Drug discovery from natural sources

Organic compounds from terrestrial and marine organisms have extensive past and present use in the treatment of many diseases and serve as compounds of interest both in their natural form and as templates for synthetic modification. Over 20 new drugs launched on the market between 2000 and 2005, originating from terrestrial plants, terrestrial microorganisms, marine organisms, and terrestrial vertebrates and invertebrates, are described. These approved substances, representative of very wide chemical diversity, together with several other natural products or their analogs undergoing clinical trials, continue to demonstrate the importance of compounds from natural sources in modern drug discovery efforts.

Synthesis and antitumor activity of the hexacyclic camptothecin derivatives

A series of hexacyclic camptothecin derivatives were synthesized to test for antitumor activity as topoisomerase I inhibitor. The strategy of synthesis was used for the formation of additional furan and dihydrofuran rings fused with 9- and 10-positions of camptothecin. All of the hexacyclic camptothecins were assayed for cytotoxicity against four human tumor cell lines, HL60, BEL-7402, HCT-116, and HeLa, and showed very impressive cytotoxicity activity in vitro. Enzyme activity of the hexacyclic camptothecins was evaluated, being equal or superior to that of SN-38. The stability of four compounds was assessed in human plasma. Two of these compounds were chosen to test for antitumor activity in vivo against Sarcoma-180. The results suggested that additional furan and dihydrofuran rings could improve the antitumor activity in vitro and vivo, though the stability of the lactone ring did not increase.

Drug discovery from medicinal plants

Current research in drug discovery from medicinal plants involves a multifaceted approach combining botanical, phytochemical, biological, and molecular techniques. Medicinal plant drug discovery continues to provide new and important leads against various pharmacological targets including cancer, HIV/AIDS, Alzheimer's, malaria, and pain. Several natural product drugs of plant origin have either recently been introduced to the United States market, including arteether, galantamine, nitisinone, and tiotropium, or are currently involved in late-phase clinical trials. As part of our National Cooperative Drug Discovery Group (NCDDG) research project, numerous compounds from tropical rainforest plant species with potential anticancer activity have been identified. Our group has also isolated several compounds, mainly from edible plant species or plants used as dietary supplements, that may act as chemopreventive agents. Although drug discovery from medicinal plants continues to provide an important source of new drug leads, numerous challenges are encountered including the procurement of plant materials, the selection and implementation of appropriate high-throughput screening bioassays, and the scale-up of active compounds.

Medicinal plants: traditions of yesterday and drugs of tomorrow

Plants have provided Man with all his needs in terms of shelter, clothing, food, flavours and fragrances as not the least, medicines. Plants have formed the basis of sophisticated traditional medicine systems among which are Ayurvedic, Unani, Chinese amongst others. These systems of medicine have given rise to some important drugs still in use today. Among the lesser-known systems of medicines are the African and Australian, Central and South American amongst others. The search for new molecules, nowadays, has taken a slightly different route where the science of ethnobotany and ethnopharmacognosy are being used as guide to lead the chemist towards different sources and classes of compounds. It is in this context that the flora of the tropics by virtue of its diversity has a significant role to play in being able to provide new leads. Nonetheless the issue of sovereignty and property rights should also be addressed in line with the Convention for Biological Diversity (CBD). This paper highlights the above, provides an overview of the classes of molecules present in plants and gives some examples of the types of molecules and secondary metabolites that have led to the development of these pharmacologically active extracts. The paper also presents some data on the use of plant products in the development of functional foods, addresses the needs for validation of plant extracts and always stressing on safety, efficacy and quality of phyto-medications.

Dendritic supramolecular assemblies for drug delivery

Dendritic supramolecular assemblies were formed in water with Reichardt's dye or the anticancer drug 10-hydroxycamptothecin and the dendritic macromolecule, ([G4]-PGLSA-OH)2-PEG3400.

Molecular characterization of two anthranilate synthase alpha subunit genes in Camptotheca acuminata

The potent anticancer and antiviral compound camptothecin (CPT) is a monoterpene indole alkaloid produced by Camptotheca acuminata. In order to investigate the biosynthetic pathway of CPT, we studied the early indole pathway, a junction between primary and secondary metabolism, which generates tryptophan for both protein synthesis and indole alkaloid production. We cloned and characterized the alpha subunit of anthranilate synthase (ASA) from Camptotheca (designated CaASA), catalyzing the first committed reaction of the indole pathway. CaASA is encoded by a highly conserved gene family in Camptotheca. The two CaASA genes are differentially regulated. The level of CaASA2 is constitutively low in Camptotheca and was found mainly in the reproductive tissues in transgenic tobacco plants carrying the CaASA2 promoter and beta-glucuronidase gene fusion. CaASA1 was detected to varying degrees in all Camptotheca organs examined and transiently induced to a higher level during seedling development. The spatial and developmental regulation of CaASA1 paralleled that of the previously characterized Camptotheca gene encoding the beta subunit of tryptophan synthase as well as the accumulation of CPT. These data suggest that CaASA1, rather than CaASA2, is responsible for synthesizing precursors for CPT biosynthesis in Camptotheca and that the early indole pathway and CPT biosynthesis are coordinately regulated.

Natural products to drugs: natural product derived compounds in clinical trials

Natural product and natural product-derived compounds that are being evaluated in clinical trials or in registration (current 31 December 2004) have been reviewed. Natural product derived drugs launched in the United States of America, Europe and Japan since 1998 and new natural product templates discovered since 1990 are discussed.

Camptothecin in sterically stabilized phospholipid micelles: A novel nanomedicine

BACKGROUND

Camptothecin (CPT) is a well-established topoisomerase I inhibitor against a broad spectrum of cancers. However, poor aqueous solubility, instability, and toxic effects to normal tissues have limited CPT clinical development. Recently, sterically stabilized micelles (SSM) composed of polyethylene glycol (PEGylated) phospholipids have been introduced as safe, biocompatible nanocarriers for the delivery of poorly water-soluble drugs. It was the aim of this study to develop and evaluate in vitro camptothecin-containing SSM (CPT-SSM) as a novel nanomedicine for parenteral administration.

METHODS

The solubilization potential, stability, and in vitro cytotoxicity of CPT in SSM were studied. Lyophilization of CPT-SSM under controlled conditions was also studied.

RESULTS

The mean size of CPT-SSM was found to be approximately 14 nm with a narrow size distribution. CPT-SSM were prepared by coprecipitation reconstitution. At a concentration of 15 mmol/L of PEGylated phospholipids where no micelle-micelle interaction was observed, CPT solubilization in SSM was 25-fold higher than CPT in buffer. We determined that CPT can be solubilized in SSM up to molar ratios of CPT/lipid = 0.0063:1. Above this critical molar ratio, heterogeneous systems of CPT-SSM and CPT self-aggregated particles were formed. CPT in SSM was at least 3 times more stable and 3-fold more cytotoxic to MCF-7 cells than CPT alone. Furthermore, CPT-SSM alone was lyophilized without additional lyoprotectants and cryoprotectants and reconstituted without any significant change in properties.

CONCLUSION

We have shown that CPT in SSM is a promising nanomedicine with improved drug solubility, stability, freeze-drying properties, and anticancer activity. It is anticipated that, because of the nanosize and steric stability of the micelles, CPT-SSM will be passively targeted to solid cancers in vivo, resulting in high drug concentration in tumors and reduced drug toxicity to the normal tissues.