M. A. Castro

Antitumor properties of podophyllotoxin and related compounds.

The lignan family of natural products includes compounds with important antineoplastic and antiviral properties such as podophyllotoxin and two of their semisynthetic derivatives, etoposide and teniposide. The latter are included in a wide variety of cancer chemotherapy protocols. Due to these biological activities, lignans, and especially cyclolignans, have been the objective of numerous studies focused to prepare better and safer anticancer drugs. The mechanism by which podophyllotoxin blocks cell division is related to its inhibition of microtubule assembly in the mitotic apparatus. However, etoposide and teniposide were shown not to be inhibitors of microtubule assembly which suggested that their antitumor properties were due to another mechanism of action, via their interaction with DNA and inhibition of DNA topoisomerase II. Other podophyllotoxin derivatives has also been reported which retained or even improved the cytotoxic activity, but these were weak inhibitors of topoisomerase II in vitro; the data revealed that such analogs exhibit a different, as yet unknown, mechanism of action. The main deficiency of these compounds is their cytotoxicity for normal cells and hence side effects derived from their lack of selectivity against tumoral cells. In this regard it is necessary to investigate and prepare new more potent and less toxic analogs, that is, with better therapeutic indices. It is well accepted from structure-activity studies in this field that the trans-lactones are more potent as antineoplastics than the cis-lactones. Not only the configuration of the D ring is an important factor for high cytotoxic activity, but also a quasi-axial arrangement of the E ring is necessary. On this basis, studies on lignans have been addressed to modify the lactone moiety and prepare analogs with heteroatoms at different positions of the cyclolignan skeleton. Our group has been working during the last few years on chemical transformations of podophyllotoxin and analogs and we have prepared a large number of cyclolignan derivatives some of which display potent antiviral, immunosuppressive and cytotoxic activities. We have reported several new cytotoxic agents with nitrogen atoms at C-7 or C-9 or at both C-7 and C-9: imine derivatives, oxime derivatives, pyrazoline-, pyrazo- and isoxazoline-fused cyclolignans. At present, we are preparing mainly new compounds by modifications of the A and E cyclolignan-rings. They are being tested on cultures of different tumoral cell lines (P-388 murine leukemia, A-549 human lung carcinoma, HT-29 human colon carcinoma and MEL-28 human melanoma) and some of them have shown an interesting and selective cytotoxicity.

Preparation and cytotoxicity of podophyllotoxin derivatives lacking the lactone ring

Abstract Several cyclolignans lacking of the lactone moiety can easily be prepared from naturally occurring lignans such as podophyllotoxin and deoxypodophyllotoxin by simple chemical transformations. Their cytotoxicity has been studied in four tumoral cell lines. Most of the compounds show similar effects in all the neoplastic systems tested, except the aldehyde 9 (methyl 9-deoxy-9-oxo-α-apopicropodophyllate) and the hydrazones 16 and 17 which show a highly selective cytotoxicity towards HT-29 human colon carcinoma. Additionally, several molecular modeling studies have been done with aldehyde 9 and the corresponding saturated aldehyde 13 in comparison with podophyllotoxin.

In vivo immunosuppressive activity of some cyclolignans

Abstract Several podophyllotoxin-related cyclolignans, either lacking the lactone ring or the methylenedioxy grouping, have been prepared and evaluated for their immunosuppressive (IMS) activity in the mouse allogeneic MLR in vitro test and in the in vivo techniques Graft vs Host Reaction (GVHR) and Skin Grafting (SG). The results obtained show that three cyclolignans fairly prevent splenomegaly in comparison with control animals and also promoted tolerance to grafting, being the first time that the in vivo IMS activity of cyclolignas is reported.

Chemoinduction of cytotoxic selectivity in Podophyllotoxin-related lignans

Lignans are widely distributed in the plant kingdom, and display a variety of biological activities which have attracted the attention of the scientific community for decades. Several representative compounds of the cyclolignan class, such as podophyllotoxin and its semisynthetic derivative, etoposide, are currently used for the clinical treatment of warts and malign neoplasms. Other cyclolignans are involved in antineoplastic and antiarthritic clinical trials. Numerous podophyllotoxin-related compounds have been prepared through modification of nearly all the positions on the cyclolignan skeleton in the search of new, more selective and less toxic anticancer drugs. Our group has been interested in the chemoinduction of drug selectivity for several years, and we have designed and prepared new podophyllotoxin derivatives by modification mainly on the C and D-rings of the podophyllotoxin skeleton. Those derivatives, bearing an electrophilic functionality at C-9, have shown, both in vitro and in vivo, a high degree of selectivity against colon carcinoma, and less cytotoxicity for other neoplastic systems and normal kidney fibroblasts. The main structural modifications found in the literature for the podophyllotoxin skeleton in the past decade, including those from our research group, are presented in this article.

Modified CoMFA methods for the analysis of antineoplastic effects of lignan analogues

Abstract We have developed a modification of CoMFA that allows the method to deal with limited conformational mobility. We tried a number of weighting schemes, including simple average, Boltzman-weighted average and a variability-weighted average of conformer CoMFA fields to produce an overall field for each molecule, the most successful being the simple average field. We have applied the method to partially rigid, C-and D-ring modified lignan natural product derivatives related to podophyllotoxin, and the biological activities of these as measured against HT29, P388 and A549 cell lines. The modified CoMFA produces more robust regressions than “standard” CoMFA. The results of the modified CoMFA appear reliable enough to suggest that at least some of these compounds act through more than one mechanism, and that the importance of each mechanism appears to depend on the cell line used.