The effect of 2- and 4-fluoro substitution on the estrogen receptor-mediated tissue localization of radioiodinated 16 alpha-iodoestradiol (16 alpha-IE2) and its 11 beta-methoxy analogue (11 beta-OMe-16 alpha-IE2) was evaluated. Electrophilic substitution of estrone or 11 beta-methoxyestrone with N-fluoropyridinium salt gave the 2- and 4-fluoro derivatives which were subsequently converted to the 3,17 beta-enol diacetate and brominated to yield exclusively the 16 alpha-bromo analogues. Epimerization gave the corresponding 16 beta-bromoestrones which were reduced to the 17 beta-hydroxy derivatives. Halogen exchange with NaI or Na[125I]I provided the A-ring fluorinated 16 alpha-iodoestradiols. The 4-F analogue exhibited higher affinity for estrogen receptors than the corresponding 2-F analogue, and these differences were more pronounced at higher incubation temperatures. Biodistribution studies in immature female rats showed that 4-fluoro substitution had only a moderate effect on receptor-mediated tissue uptake of the parent molecules whereas 2-fluoro substitution resulted in strongly diminished tissue specificity. The lower target selectivity of the 2-F, compared to the 4-F, analogue correlates to some extent with their different receptor binding properties; however, the rate of catabolism may also be involved. Differences in blood clearance further accentuated the localization properties to yield particularly high uterus to blood ratios in the case of the 4-F-11 beta-OMe-16 alpha-IE2, suggesting the potential of the analog labeled with 123I as a radiopharmaceutical for receptor imaging in nuclear medicine. The isopotential maps of the fluorinated steroids, obtained via semiempirical computer modeling on the molecular structures, show striking differences between the 4-F and 2-F derivatives reflecting their varying biological properties.

In order to obtain information concerning the mechanism(s) of metallo-phthalocyanine (MePcS4) photosensitized damage of DNA constituents, the EPR-spin trapping method in conjunction with liquid chromatography was used to study thymidine (dThd) free radicals formed during photosensitization or exposure to gamma-radiation in solution. Under specified conditions two dThd free radical species, 5-hydroxy-5,6-dihydrothymidine-6-yl and 6-hydroxy-5,6-dihydrothymidine-5-yl, were formed both during exposure to ionizing radiation and photosensitization. These results imply that identical reactive intermediates (*OH radicals) are involved in the radiolytic and photosensitized oxidation of dThd. A light-dependent, Fenton-type mechanism is proposed to explain the generation of hydroxyl radicals during MePcS4 photosensitization.