Synthesis and Biological Evaluation of Radioiodinated Phospholipid Ether Stereoisomers

Radioiodinated phospholipid ethers have shown the remarkable ability to selectively accumulate in a variety of animal tumors as well as in human tumor xenografts. It has been suggested that this tumor avidity may arise as a consequence of metabolic differences between tumor and corresponding normal tissue. One such compound, 1-0-[12-(m-iodophenyl)dodecyl]-2-0-methyl-rac-glycero-3-phosphocholine (NM-294), contains a chiral center at the sn-2 position. The unnatural S- and natural R-enantiomers (4 and 5, respectively) of NM-294 were synthesized in order to provide further information on the mechanism(s) responsible for the tumor avidity of phospholipid ethers. In vitro cytotoxicity studies demonstrated a lack of stereospecificity. Biodistribution studies in rats bearing the Walker 256 tumor demonstrated the S- and R-isomers to have similar tissue uptake at 24 and 48 h after administration. Tumor-to-blood ratios at 24 h were 11.1 and 11.0 for the S- and R-isomers, respectively. In addition, γ-camera scintigrams of tumor-bearing rats at various time points after iv administration of the S- and R-isomers did not show any qualitative differences in the distribution of radioactivity. Prior studies have shown that rac-NM-294 was not a substrate for phosphatidylcholine specific phospholipase C, but was a substrate for two forms of phospholipase D (PLD). Therefore, metabolism studies with 4 and 5 with various forms of PLD were performed. PLD from cabbage demonstrated a degree of stereoselectivity. In the presence of 1% ethanol, the R-isomer was metabolized to the greatest extent, followed by rac-NM-294 and the S-isomer. PLD isolated from Streptomyces chromofuscus failed to demonstrate any stereoselectivity. The results suggest that the mechanism(s) of retention of these compounds in tumors may not involve a highly stereoselective component.