Potential tumor or organ-imaging agents, 24. Radioiodinated pregnenolone esters

Synthesis, biological evaluation, and baboon PET imaging of the potential adrenal imaging agent cholesteryl-p-[18F]fluorobenzoate

Cholesteryl-p-[18F]fluorobenzoate ([18F]CFB) was investigated as a potential adrenal positron emission tomography (PET) imaging agent for the diagnostic imaging of adrenal disorders. We describe the synthesis, biodistribution, adrenal autoradiography, and baboon PET imaging of [18F]CFB. The synthesis of [18F]CFB was facilitated by the use of a specially designed microwave cavity that was instrumental in effecting 70-83% incorporation of fluorine-18 in 60 s via [18F]fluoro-for-nitro exchange. Tissue distribution studies in mature female Sprague-Dawley rats showed good accumulation of [18F]CFB in the steroid-secreting tissues, adrenals and ovaries, at 1 h postinjection. The effectiveness of [18F]CFB to accumulate in diseased adrenals was shown through biodistribution studies in hypolipidemic rats, which showed a greater than threefold increase in adrenal uptake at 1 h and increased adrenal/liver and adrenal/kidney ratios. Analysis of the metabolites at 1 h in the blood, adrenals, spleen, and ovaries of hypolipidemic and control rats showed the intact tracer representing greater than 86%, 93%, 92%, and 82% of the accumulated activity, respectively. [18F]CFB was confirmed to selectively accumulate in the adrenal cortex versus the adrenal medulla by autoradiography. Normal baboon PET imaging with [18F]CFB effectively showed adrenal localization as early as 15 min after injection of the tracer, with enhanced adrenal contrast seen at 60-70 min. These results suggest that [18F]CFB may be useful as an adrenal PET imaging agent for assessing adrenal disorders.

Diagnostic and therapeutic potential of poly(benzyl L-glutamate)

Poly(benzyl L-glutamate) (PBLG) microcapsules, prepared by a solvent evaporation technique for intravenous injection, are evaluated for their potential use in diagnostic computed tomographic enhancement of liver images. The smaller microcapsules, < 3 microns, loaded with a radiopaque contrast material, ethyl iopanoate (IOPAE), produced prolonged opacification of the liver when delivered intravenously. In vivo tissue distribution studies of PBLG-131I-IOPAE (5 microCi/rat, iv) showed that liver had the highest uptake (percent of injected dose/g of tissue) among other organs 24 h postinjection. An in vitro estrogen receptor assay in pig uteri indicated that PBLG conjugated with estrone did not interfere with estrogen receptor affinity, suggesting the estrogen therapy potential of PBLG-estrone.

Potential tumor or organ imaging agents--31. Radioiodinated sterol benzoates and carbamates

A series of radioiodinated benzoate and carbamate esters of cholesterol and pregnenolone wherein the acyl moiety served as the carrier for radioiodine was synthesized and evaluated as potential imaging agents for the adrenal cortex. 2,6-Dimethyl-3-iodobenzoyl and N-(4-iodophenyl) carbamoyl groups were chosen as the acyl functionality in an attempt to provide esters resistant to in vivo hydrolysis. Tissue disposition studies in rats revealed that their biodistribution was determined by the attached sterol carrier-the cholesterol esters demonstrated significant uptake at 24 h in the adrenal whereas the corresponding pregnenolone derivatives showed only slight affinity for steroid-secreting tissues at this time.

Potential tumor- or organ-imaging agents. 28. Radioiodinated esters of cholesterol and pregnenolone

Previous studies had shown radioiodinated esters of cholesterol and pregnenolone to accumulate in steroid-secreting tissues of the rat. This was particularly true for radioiodinated iopanoate esters. The present study was undertaken to examine the effect of the iopanoyl amino group on the tissue distribution of these esters. While the tissue distribution profiles for cholesteryl iopanoate and the desamino analog (III) were somewhat comparable, such was not the case for the corresponding esters of pregnenolone. Moreover, this subtle structural change of removing the amino group was observed to affect the in vivo stability of the esters to hydrolysis. This conclusion is in accordance with the observation that the tissue distribution profiles for the free acids I and II are not significantly different from each other. These studies serve to demonstrate that relatively minor modifications of the acyl moiety have a profound effect on both the uptake and distribution of these sterol esters in various tissues.