Hiroaki Okuno

Novel approaches to prodrugs of anticancer diaminodichloroplatinum(II) complexes activated by stereoselective enzymatic ester hydrolysis

A series of dichloro(ethylenediamine)-type platinum complexes bearing ester-, amide- and ether-bonded alkyl straight chains (C2-C18) was prepared as a model for the prodrug of cis-diamminedichloroplatinum (CDDP). Evaluation of cytotoxic activity of the complexes against the S-180 cell line proved that their enzymatic hydrolysis in the cells lowered the permeability to the cell membranes resulting in an increase of pharmacological activity. Optically active isomers of dichloro(N-2(2-phenylpropanoyloxy)ethyl)-2,3-diaminopropanamide )platinum(II) were prepared. They were re-activated stereoselectively, and a tenfold difference in activity was observed based on the differences in chiral discrimination by enzymes.

Difference between cancer cells and the corresponding normal tissue in view of stereoselective hydrolysis of synthetic esters

This study has aimed at taking information necessary for design of anticancer prodrugs modified with chiral acyl group, especially about the effect of chirality of the acyl group on its enzymic removal in specific cells. Thus, 13 species of chiral esters were synthesized and stereoselectivity in their enzymic hydrolysis was investigated with six cancer cell lines, solid tumors, and the corresponding normal tissues. Cultured cancer cells from rat liver, pancreas, and muscle hydrolyzed the R enantiomer of (+/-)-ethyl 2-methoxy-2-phenylacetate (3c) more preferentially than its antipode, whereas this stereoselectivity was reversed in the reaction by homogenate of the corresponding normal tissue of rat. The difference in stereoselectivity between cancer cells and normal tissue was also found in the hydrolysis of other esters including those of actual anticancer agents, p-hydroxyaniline mustard and 5-fluorouridine. The investigation was expanded to real tumor to show that the degree of stereoselectivity or the hydrolytic activity was significantly different between a human brain tumor and its surrounding normal tissue for such substrates as (+/-)-ethyl 2-phenoxypropanoate and N-trifluoroacetylphenylalaninate. The esterases of rat liver cancer cells (Anr4) and normal rat liver gave different band patterns in active staining after gel electrophoresis. The enzymes were fractionated by ion exchange column chromatography and then tested on their stereoselectivity against (+/-)-3c. Comparison of the results and electrophoretograms of the fractions suggests that esterases with different stereoselectivity are expressed in different ways by normal and cancer cells. These results show that stereoselectivity in enzymic hydrolysis of some synthetic chiral esters is different between cancer and normal cells, leading to the possibility that specific activation of ester-type anticancer prodrugs in cancer cells would be controlled by the chiral structure of the acyl group.

Preparation, structure and cytotoxicity of cis-diammineplatinum(II) dinuclear complexes with 1-alkyluracil and imidate ligands ☆

Abstract A series of platinum dinuclear complexes with l-alkyluracil, cis -[Ph 2 (PtU) 2 (NH 3 ) 4 ] (NO 3 ) 2 , cis-[Pt 2 (n-BuU) 2 (NH 3 ) 4 ](NO 3 ) 2 , cis-[Ph 2 (BzlU) 2 (NH 3 ) 4 ](NO 3 ) 2 and cis-[Pt 2 (NaphCH 2 U) 2 (NH 3 ) 4 ](NO 3 ) 2 (head-to-head, where U=uracil), and with imide ligands, cis-[Ph 2 (SI 2 (NH 3 ) 4 ](NO 3 ) 2 (head-to-head), cis-[Pt 2 (DMGI) 2 (NH 3 ) 4 ](NO 3 ) 2 (head-to-head), cis-[Pt 2 (DMGI) 2 (NH 3 ) 4 ](NO 3 ) 2 (head-to-tail) and cis-[Pt 2 (DMGI) 2 (NH 3 ) 4 ] (NO 3 ) 2 (head-to-head, where SI = succinimidate, EMGI = 3-ethyl-3-methylglutarimidate and DMGi=3,3.dimethylglutarimidate) was synthesized, as well as platinum mononuelear complexes, cis-[PtCl(SI)(NH 3 ) 2 ] and cis- [Pt(SI) 2 (NH 3 ) 2 ]. The isomers of the dinuclear complexes (head-to-head and head-to-tail forms) were obtained separately by fractional recystallizations. Crystal structures of cis-[Pt 2 (SI) 2 (NH 3 ) 4 ](NO 3 ) 2 (head-to-head), cis-[Pt 2 (DMGI) 2 (NH 3 ) 4 ](NO 3 ) 2 (head-to-head) and cis-[Pt 2 (EMGI) 2 (NH 3 ) 4 ](NO 3 ) 2 (head-to-head) were determined by X-ray diffraction analysis. Cytotoxic activity was evaluated by the IC 50 value using mouse sarcoma S-180 cells. Most head-to-head complexes are considerably active, while the corresponding head-to-tail analogues are inactive. The active complexes, in general, release ligands in saline at 37°C, and a relationship between hydrophobicity of the complexes and the IC 50 value has been shown.

SYNTHESIS, STRUCTURE AND CYTOTOXIC ACTIVITY OF PLATINUM(II) DIMERS WITH BRIDGING IMIDATE LIGANDS IN HEAD-TO-HEAD ARRANGEMENT

Abstract The cis-diammineplatinum(II) head-to-head dimer complexes with imidate ligands, [Pt2(NH3)4(SI)2](NO3)2 · H2O, [Pt2(NH3)4-(EMGI)2] (NO3)2 and [Pt2(NH3)4(DMGI)2](NO3)2 (where SI = succinimidate, EMGI = 3-ethyl-3-methylglutarimidate and DMGI = 3,3-dimethylglutarimidate), are prepared from aqueous solutions of platinum(II) in the presence of the corresponding imides. These complexes form a pair of dimers within the crystals stabilized by hydrogen bonds as shown by X-ray structural analyss. They exhibit high antitumor activities against S180 and L1210 cancer cell lines.

Effect of fluorine substitution of α-and β-hydrogen atoms in ethyl phenylacetate and phenylpropionate on their stereoselective hydrolysis by cultured cancer cells

Abstract ( ±) -Ethyl 2-fluoro-2-phenylacetate was stereoselectively hydrolyzed by cultured cells of several rat cancer cell lines to give the carboxylic acid rich in the R enantiomer. The stereoselectivity increased for ( ±)-ethyl 2-fluoro-2-phenylpropionate ( 2b ) with all present cell lines and for ( ±)-ethyl 2-phenyl-3,3,3-trifluoropropionate (3b) with rat hepatoma McA-RH7777 cell line. The stereoselectivity was different for the different cell lines, as McA-RH7777 cells preferred ( R ) - 2b in contrast with the preference towards ( S ) - 2b by other cells such as ras oncogenetransformed rat liver Anr4 cells. These stereoselectivities were different from those for non-fluorinated ( ±)-ethyl 2-phenylpropionate. Thus fluorine atoms are recognized by ester hydrolases of cancer cells, and fluorine substitution on the acyl group will be useful for making estertype anticancer prodrugs more specific to cancer cells.