Podophyllotoxin is an important and much sought after antimitotic natural lead compound, since it paved the way for three hemisynthetic derivatives of podophyllotoxin, e.g., etoposide, teniposide and etopophos, which are widely used as anticancer drugs and show good clinical effects against several types of neoplasms. Although the publication of the recent reviews by Gordaliza in 2004 and You in 2005, which covered the literatures concerning podophyllotoxin until the early part of 2003, there have been significant number of works carried out on podophyllotoxin recently. Therefore, this review presents up-to-date coverage of podophyllotoxin in regard to hemisynthesis, biosynthesis, biological activities, mode of action and structure-activity relationship.

A series of enantiomerically pure polypyridyl ruthenium(II) complexes, delta- and lambda-[Ru(bpy)2 (HPIP)](PF6)2 (delta-1 and lambda-1; bpy=2,2'-bipyridine, HPIP = 2-(2-hydroxyphenyl)imidazo[4,5-f][1,10]phenanthroline), delta and lambda-[Ru(bpy)2(HNAIP)](PF6)2 (delta-2 and lambda-2; HNAIP = 2-(2-hydroxy-1-naphthyl)imidazo[4,5-f][1,10]phenanthroline), delta- and lambda-[Ru(bpy)2 (HNOIP)](PF6)2 (delta-3 and lambda-3; HNOIP = 2-(2-hydroxy-5-nitrophenyl)imidazo[4,5-f][1,10]phenanthroline), and delta- and lambda-[Ru(bpy)2(DPPZ)](PF6)2 (delta-4 and lambda-4; DPPZ= dipyridophenazine), have been synthesized. Binding behavior of these chiral complexes to calf thymus DNA (CT-DNA) has been investigated by electronic absorption, steady-state emission, and circular dichroism spectroscopies, as well as by viscosity measurements and equilibrium dialysis binding studies. Several points came from the results. (1) The DNA-binding properties were distinctly different for the [Ru(bpy)2L]2+ (L=HPIP, HNAIP, HNOIP) series of ruthenium(II) complexes, which indicates that the photophysical behavior of the complexes on binding to DNA can be modulated through ligand design. (2) Different binding rates of individual enantiomers of complexes 1 and 4 to DNA were observed through dialysis experiments. The lambda enantiomer bound more rapidly than the lambda enantiomer and their different intercalative binding geometries were suggested to be responsible. (3) Both delta-2 and lambda-2 bound weakly to CT-DNA; delta-2 may bind through a partial intercalation mode, whereas lambda-2 may bind in the DNA groove. (4) There was no noticeable enantioselectivity for complexes 1, 3, and 4 on binding to CT-DNA. Both of their enantiomers can intercalate into DNA base pairs. It is noted that delta-3 and lambda-3 exhibited almost identical spectral changes on addition of CT-DNA, and a similar binding manner of the isomers to the double helix was proposed.

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Podophyllotoxin (PPT), as well as its congeners and derivatives, exhibits pronounced biological activities, especially antineoplastic effects. Its strong inhibitory effect on tumor cell growth led to the development of three of the most highly prescribed anticancer drugs in the world, etoposide, teniposide, and the water-soluble prodrug etoposide phosphate. Their clinical success as well as intriguing mechanism of action stimulated great interest in further modification of PPT for better antitumor activity. The C-4 position has been a major target for structural derivatization aimed at either producing more potent compounds or overcoming drug resistance. Accordingly, numerous PPT derivatives have been prepared via hemisynthesis and important structure–activity relationship (SAR) correlations have been identified. Several resulting compounds, including GL-331, TOP-53, and NK611, reached clinical trials. Some excellent reviews on the distribution, sources, applications, synthesis, and SAR of PPT have been published. This review focuses on a second generation of new etoposide-related drugs and provides detailed coverage of the current status and recent development of C-4-modified PPT analogs as anticancer clinical trial candidates.

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