Synthesis and in vitro cytotoxicity of C(20)(RS)-camptothecin analogues modified at both B (or A) and E ring

Natural product inspired leads in the discovery of anticancer agents: an update

Natural products have emerged as major leads for the discovery and development of new anti-cancer drugs. The plant-derived anti-cancer drugs account for approximately 60% and the quest for new anti-cancer agents is in progress. Anti-cancer leads have been isolated from plants, animals, marine organisms, and microorganisms from time immemorial. The process of semisynthetic modifications of the parent lead has led to the generation of new anti-cancer agents with improved therapeutic efficacy and minimal side effects. The various chemo-informatics tools, bioinformatics, high-throughput screening, and combinatorial synthesis are able to deliver the new natural product lead molecules. Plant-derived anticancer agents in either late preclinical development or early clinical trials include taxol, vincristine, vinblastine, topotecan, irinotecan, etoposide, paclitaxel, and docetaxel. Similarly, anti-cancer agents from microbial sources include dactinomycin, bleomycin, mitomycin C, and doxorubicin. In this review, we highlighted the importance of natural products leads in the discovery and development of novel anti-cancer agents. The semisynthetic modifications of the parent lead to the new anti-cancer agent are also presented. Further, the leads in the preclinical settings with the potential to become effective anticancer agents are also reviewed.Communicated by Ramaswamy H. Sarma.

Topoisomerase 1B poisons: Over a half-century of drug leads, clinical candidates, and serendipitous discoveries

Topoisomerases are DNA processing enzymes that relieve supercoiling (torsional strain) in DNA, are necessary for normal cellular division, and act by nicking (and then religating) DNA strands. Type 1B topoisomerase (Top1) is overexpressed in certain tumors, and the enzyme has been extensively investigated as a target for cancer chemotherapy. Various chemical agents can act as "poisons" of the enzyme's religation step, leading to Top1-DNA lesions, DNA breakage, and eventual cellular death. In this review, agents that poison Top1 (and have thus been investigated for their anticancer properties) are surveyed, including natural products (such as camptothecins and indolocarbazoles), semisynthetic camptothecin and luotonin derivatives, and synthetic compounds (such as benzonaphthyridines, aromathecins, and indenoisoquinolines), as well as targeted therapies and conjugates. Top1 has also been investigated as a therapeutic target in certain viral and parasitic infections, as well as autoimmune, inflammatory, and neurological disorders, and a summary of literature describing alternative indications is also provided. This review should provide both a reference for the medicinal chemist and potentially offer clues to aid in the development of new Top1 poisons.

The structure-activity relationships of A-ring-substituted aromathecin topoisomerase I inhibitors strongly support a camptothecin-like binding mode

Aromathecins are inhibitors of human topoisomerase I (Top1). These compounds are composites of several heteroaromatic systems, namely the camptothecins and indenoisoquinolines, and they possess notable Top1 inhibition and cytotoxicity when substituted at position 14. The SAR of these compounds overlaps with indenoisoquinolines, suggesting that they may intercalate into the Top1-DNA complex similarly. Nonetheless, the proposed binding mode for aromathecins is purely hypothetical, as an X-ray structure is unavailable. In the present communication, we have synthesized eight novel series of A-ring-substituted (positions 1-3) aromathecins, through a simple, modular route, as part of a comprehensive SAR study. Certain groups (such as 2,3-ethylenedioxy) moderately improve Top1 inhibition, and, often, antiproliferative activity, whereas other groups (2,3-dimethoxy and 3-substituents) attenuate bioactivity. Strikingly, these trends are very similar to those previously observed for the A-ring of camptothecins, and this considerable SAR overlap lends further support (in the absence of crystallographic data) to the hypothesis that aromathecins bind in the Top1 cleavage complex as interfacial inhibitors in a 'camptothecin-like' pose.

Liposome drugs' loading efficiency: a working model based on loading conditions and drug's physicochemical properties

Remote loading of liposomes by transmembrane gradients is one of the best approaches for achieving the high enough drug level per liposome required for the liposomal drug to be therapeutically efficacious. This breakthrough, which enabled the approval and clinical use of nanoliposomal drugs such as Doxil, has not been paralleled by an in-depth understanding that allows predicting loading efficiency of drugs. Here we describe how applying data-mining algorithms on a data bank based on Barenholz's laboratory's 15 years of liposome research experience on remote loading of 9 different drugs enabled us to build a model that relates drug physicochemical properties and loading conditions to loading efficiency. This model enables choosing candidate molecules for remote loading and optimizing loading conditions according to logical considerations. The model should also help in designing pro-drugs suitable for remote loading. Our approach is expected to improve and accelerate development of liposomal formulations for clinical applications.

Design, synthesis, and biological evaluation of 14-substituted aromathecins as topoisomerase I inhibitors

The aromathecin or "rosettacin" class of topoisomerase I (top1) inhibitors is effectively a "composite" of the natural products camptothecin and luotonin A and the synthetic indenoisoquinolines. The aromathecins have aroused considerable interest following the isolation and total synthesis of 22-hydroxyacuminatine, a rare cytotoxic natural product containing the 12 H-5,11a-diazadibenzo[ b, h]fluoren-11-one system. We have developed two novel syntheses of this system and prepared a series of 14-substituted aromathecins as novel antiproliferative topoisomerase I poisons. These inhibitors are proposed to act via an intercalation and "poisoning" mechanism identical to camptothecin and the indenoisoquinolines. Many of these compounds possess greater antiproliferative activity and anti-top1 activity than the parent unsubstituted compound (rosettacin) and previously synthesized aromathecins, as well as greater top1 inhibitory activity than 22-hydroxyacuminatine. In addition to potentially aiding solubility and localization to the DNA-enzyme complex, nitrogenous substituents located at the 14-position of the aromathecin system have been proposed to project into the major groove of the top1-DNA complex and hydrogen-bond to major-groove amino acids, thereby stabilizing the ternary complex.

In vitro pharmacodynamics of CKD-602 in HT-29 cells

CKD-602 (7-[2-(N-isopropylamino)ethyl]-(20S)-camptothecin) is a recently-developed synthetic camptothecin analogue and currently under clinical development by Chong Kun Dang Pharm (Seoul, Korea). CKD-602 showed potent topoisomerase inhibitory activity in vitro and broad antitumor activity against various human tumor cells in vitro and in vivo in animal models. This study describes the pharmacodynamics of the immediate and delayed cytotoxicity induced by CKD-602 in a human colorectal adenocarcinoma cell line, HT-29, and its intracellular drug accumulation by HPLC. The present study was designed to address whether the higher activity of CKD-602 with prolonged exposure is due to delayed exhibition of cytotoxicity and/or an accumulation of antiproliferative effect on continuous drug exposure. The drug uptake study was performed to determine whether the delayed cytotoxicity is due to a slow drug accumulation in cells. CKD-602 produced a cytotoxicity that was exhibited immediately after treatment (immediate effect) and after treatment had been terminated (delayed effect). Both the immediate and delayed effects of CKD-602 showed a time dependent decrease in IC50 values. Drug uptake was biphasic and the second equilibrium level was obtained as early as at 24 hr, indicating that the cumulative and delayed antitumor effects of CKD-602 were not due to slow drug uptake. On the other hand, CKD-602 treatment was sufficient to induce delayed cytotoxicity after 4 hr, however, longer treatment (>24 hr) enhanced its cytotoxicity due to the intracellular accumulation of the drug, which requires 24 hr to reach maximum equilibrium concentration. In addition, Cn x T=h analysis (n=0.481) indicated that increased exposure times may contribute more to the overall antitumor activity of CKD-602 than drug concentration. Additional studies to determine the details of the intracellular uptake kinetics (e.g., concentration dependency and retention studies) are needed in order to identify the optimal treatment schedules for the successful clinical development of CKD-602.

Preformulation studies of a novel camptothecin anticancer agent, CKD-602: physicochemical characterization and hydrolytic equilibrium kinetics

(20S)-7-(2-isopropylamino)ethylcamptothecin.HCl (CKD-602), a new camptothecin (CPT) anticancer agent, is a pale yellowish crystalline compound. DSC thermogram exhibited a melt endotherm near 270 degrees C, and CKD-602 was found to be slightly hygroscopic. The solubility of CKD-602 in deionized water was 8.22 mg/ml, and two pK(a) values were measured to be 2.32 and 9.15, respectively. A pH-dependent partition coefficient behavior in octanol-buffer was observed. CKD-602 in solid state was stable over the range of temperature and humidity, but decomposed slightly by light. The hydrolysis of CKD-602 occurred reversibly and rapidly in aqueous buffer solutions. The conversion rate constants (k(f): from the lactone to the carboxylate and k(r): from the carboxylate to the lactone) and the final equilibrium ratio (K(eq)) between two species were dependent on the pH of aqueous solutions.

Synthesis and in vitro cytotoxicity of hexacyclic camptothecin analogues

A series of C(7)-N-alkylaminoethyl-C(10), C(11)-methylenedioxy- and ethylenedioxy-camptothecin (3a-g, 4a-b) were prepared. Their syntheses and in vitro cytotoxicity were reported. Among 15 derivatives, 3a and 3b showed more potent cytotoxicity than Camptothecin, especially in CAOV-3 cell line.