Synthesis of high affinity fluorine-substituted ligands for the androgen receptor. Potential agents for imaging prostatic cancer by positron emission tomography

Abstract

We have prepared nine androgens substituted with fluorine at C-16 or C-20 to evaluate their potential, as positron emission tomographic (PET) imaging agents for prostatic cancer when labeled with the positron emitting radionuclide fluorine-18 (t1/2 = 110 min). These compounds represent members from the following classes of androgens: testosterone (T), 5 alpha-dihydrotestosterone (DHT), 7 alpha-methyl-19-nortestosterone (MNT), mibolerone (Mib), and metribolone (R1881). All of these compounds were prepared by functionalization of suitable androgen precursors, and the synthetic routes were developed to allow the introduction of fluorine by a fluoride ion displacement reaction late in the synthesis, as is required for the preparation of these compounds in fluorine-18 labeled form. We have also prepared four androgens in which the C-3 carbonyl or 17 beta-hydroxyl groups are replaced by fluorine. Most of the fluorine-substituted androgens show high affinity for the androgen receptor (AR), although fluorine substitution lowers their affinity by a small factor. None of the androgens where fluorine replaces oxygen functions at C-3 or C-17 have substantial affinity for AR. Derivatives of the natural androgens (T and DHT) as well as MNT have little affinity for other steroid hormone receptors (progesterone and mineralocorticoid receptors), whereas the Mib and R1881 derivatives have somewhat greater heterologous binding. With sex steroid binding protein, a human serum binding protein, the pattern of binding affinities is nearly the reverse, with derivatives of Mib, R1881 and MNT having low affinity, and DHT and T, high affinity. From these fluorine-substituted compounds, we can select several whose preparation in fluorine-18 labeled form for further tissue distribution studies is merited.