A novel 99mTc-labeled testosterone derivative as a potential agent for targeting androgen receptors

Small Molecules for Active Targeting in Cancer

For the purpose of this review, active targeting in cancer research encompasses strategies wherein a ligand for a cell surface receptor expressed on tumor cells is used to deliver a cytotoxic or imaging cargo. This area of research is more than two decades old, but in those 20 and more years, how many receptors have been studied extensively? What kinds of the ligands are used for active targeting? Are they mostly naturally occurring molecules such as folic acid, or synthetic substances developed in campaigns for medicinal chemistry efforts? This review outlines the most important receptor or ligand combinations that have been used in active targeting to answer these questions, and therefore to address the most important one of all: is research in active targeting affording diminishing returns, or is this an area for which the potential far exceeds progress made so far?

Synthesis and preliminary bioevaluation of 99mTc(CO)3-17α-triazolylandrost-4-ene-3-one derivative prepared via click chemistry route

Azolyl steroids are known to manifest antiprostate cancer and antiandrogenic activities. These azolyl steroids have been shown to express affinity toward androgen receptors (ARs) overexpressed on LNCaP (human prostate adenocarcinoma) cell line. Hence, suitably derivatized azolyl steroids can be envisaged as potential vectors for targeting overexpression of ARs in prostate cancer. In the present study, testosterone has been derivatized to 17α-azidoandrost-4-ene-3-one using microwave-mediated azidation of the mesylate. Subsequently, a facile one-pot Cu(I)-catalyzed Click reaction was carried out to synthesize (99m)Tc(CO)(3)-labeled 17α-triazolylandrost-4-ene-3-one, which was characterized by HPLC. The chemical characterization of (99m)Tc(CO)(3)-17α-triazolylandrost-4-ene-3-one was carried out by preparing its corresponding rhenium complex using [NEt(4)](2)[Re(CO)(3)Br(3)] precursor. The radiolabeled complex could be prepared in >95% radiochemical yield as determined by HPLC. In vitro studies of (99m)Tc(CO)(3)-17α-triazolylandrost-4-ene-3-one complex in LNCaP cell lines overexpressing ARs showed binding of 4.95%±1.2%, with inhibition of 8%±0.9%. In vivo biodistribution studies in male Wistar rats have shown uptake in the prostate to the extent of 0.48%±0.19% injected dose/g at 1 hpi and retention therein till 3 hpi. The present study demonstrates a novel and facile one-pot reaction for preparation of (99m)Tc-labeled 17α-triazolylandrost-4-ene-3-one complex using Click chemistry. The corresponding Re-analog has been prepared for purpose of comparative characterization with the (99m)Tc-labeled complex. The radiosynthetic strategy described in this article can be further extended toward preparation of radiolabeled complexes of other triazolyl steroidal derivatives.

Steroid-coupled liposomes for targeted delivery to tumor

The steroidal receptors play a key role in protein synthesis and maintain the homeostasis in normal and diseased state, including tumorigenesis at the target tissues when overactivated. Thus steroidal receptors may act as potential targets for selective delivery of different therapeutic agents as they are overexpressed by a number of endocrinal tumors. The selective delivery of these agents may be a better treatment strategy for endocrinal cancer as it may also result in cytosolic and nuclear delivery of cytotoxic agents. In this review, the targeting potential of steroidal receptors for the drug or bioactive(s) delivery is discussed. The ligands that have been proven to be effective for specific steroidal receptors can be used as vectors for carrying the drug or drug-delivery system to the desired site of drug action in an optimum concentration. This strategy will not only minimize the undesired side effects associated with nonspecific delivery of drug, but will also maximize the drug utilization. Ligand-conjugated liposomes as a carrier of bioactives prevent passive diffusion of the encapsulated drug to normal cells, increase the time of circulation and reduce the undesirable side effects of a drug.

Synthesis and characterization of nonsteroidal-linked M(CO)(3)+ (M = 99mTc, Re) compounds based on the androgen receptor targeting molecule flutamide

Androgen receptors are overexpressed in most primary and metastatic prostate cancers. A series of single photon emission computed tomography imaging agents (SPECT) utilizing the organometallic radioactive imaging species, fac-99mTc(OH(2))(3)(CO)(3)+, were prepared on the basis of the structure of Flutamide, a potent nonsteroidal antiandrogen prostate cancer drug. Novel bifunctional chelate-linked Flutamide analogues were prepared using a newly developed universal alkylating reagent, 2-bromo-N-[4-nitro-3-(trifluoromethyl)phenyl]-acetamide, 1. From compound 1, several ligands (i.e., cysteine 2, histidine 5, imidazole 3) were conjugated to the flutamide derivative to yield targeting ligands capable of either tridentate or monodentate coordination in a "2 + 1" complex. fac-Re(CO)(3)+ complexes were prepared and characterized with the functionalized conjugates to yield fac-Re(CO)(3)(2-amino-3-(1-(2-(4-nitro-3-(trifluoromethyl)phenylamino)-2-oxoethyl)-1H-imidazol-4-yl) propanoate), 4, fac-Re (CO)(3)(2-(S-cysteinyl)-N-[4-nitro-3-(trifluoromethyl) phenyl]-acetamide), 6, and fac-Re(CO)(3)(picolinate)(2-(1H-imidazol-1-yl)-N-[4-nitro-3-(trifluoromethyl)phenyl]-acetamide), 7. The corresponding radioactive 99mTc analogues were prepared and stability studies of the radioactive compounds were also conducted.