to support tumor growth. this phenomenon is called antiandrogen withdrawal syndrome (aws) and is one of the major drawbacks of existing antiandrogens. aws is defined as tumor regression or symptomatic relief observed upon discontinuation of the antiandrogen therapy. the mechanism for this is not fully understood but current theories include alterations of the ar gene, coregulator proteins and/or signal transduction pathways. this antiandrogen resistance may also be linked to the relative weakness of current antiandrogens as they have an affinity 50 times or more lower than that of dht for the ar. this may also explain why compensatory ar overexpression is often observed. androgen receptor gene mutations ar gene mutations in the lbd that alter ligand specificity and/or functional activity exist and are thought to contribute to the conversion of some ar antagonists into agonists, which explains the paradoxical temporary improvement sometimes observed in patients when antiandrogen therapy is stopped. these mutations can have great effect on the antagonist activities of current small molecule antiandrogens and make them less efficient in blocking ar function via indirect modulation from inside of the lbp. recent studies with circulating tumor cells, suggest that the mutation frequency is higher than previously assumed based on tumor biopsies. the t877a, w741l and w741c mutations are examples of known ar lbd mutations. the lncap prostate cancer cell line expresses ar with a t877a point mutation that causes proliferation in the presence of the antiandrogens hydroxyflutamide and cyproterone acetate. this mutation has also been discovered in patients with antiandrogen withdrawal syndrome being treated with these compounds. in another study, bicalutamide treatment of lncap cells resulted in two lbd mutations, w741l and w741c, causing bicalutamide to acquire agonist activity to both mutant ars. the w741l mutation generates additional space such that the sulfonyl-linked phenyl ring of bicalutamide is accommodated at the location of the missing indole ring of w741. in non-mutant ar, the presence of the w741 side chain probably forces bicalutamide to protrude out thus precluding the active position of h12 on the ar receptor. however, hydroxyflutamide worked as an antagonist for w741 mutant ars. this concurs with the theory that flutamide and nilutamide antagonize ar through the mechanism of “passive antagonism”, as they are of a more modest size then bicalutamide. these drugs may therefore be effective as a second-line therapy for refractory prostate cancer previously treated with bicalutamide. current status n-terminal domain antagonists antagonists of the n-terminal domain (ntd) of the ar have been proposed to overcome the limitations of current antiandrogens regarding mutant ars, by directly blocking ar function from protein surface, outside of the lbp. this direct blockade is thought to provide a more efficient strategy to avoid or overcome abnormal ar action during aws, as well as allowing for more flexibility in structural modification without the space limitations of the rigid lbp. steroid receptors have similarities in gene sequences and protein structures, leading often to functional crosstalk among steroid receptors. one of the criteria for ar ntd antagonists is to achieve high degree of specificity for the ar. it is though important to realize that ar specificity does not necessarily translate in vivo, since ntd antagonists may also interact with protein targets other than ar. ligand binding domain as target site ar activation requires the formation of a functional activation function 2 (af2) region in ar lbd that mediates the interactions between ar and various transcription cofactors. therefore, most of the research on ntd ar antagonists focuses on peptides that may directly block the af2 in ar lbd from protein surface. even in bound mutant ar, ntd antagonists would be able to block the af2 function via direct surface interaction, regardless of the ligand bound. research on these ntd antagonists are usually carried out by affinity screening of phage display libraries that express random peptides containing various signature motifs. ars seem to have a distinct preference for ‘fxxlf’ type of binding motifs (where f = phenylalanine, l = leucine, and x = any amino acid residue), whereas other nuclear receptors have a highly similar binding mechanism for ‘lxxll’ type of binding motifs. this provides a unique opportunity for the development of ar-specific peptides. even though small molecule antagonists and ntd antagonist targeting af2 surface differ in binding sites, they both inhibit ar function by disrupting af2 function. therefore, mechanistically, these ntd antagonists may also be classified as ‘af2 antagonists’. figure 8 sintokamide a n-terminal domain as target site functionally, ar ntd plays the primary role in regulating target gene transcription activation and mediating various receptor-protein and intra-receptor n-terminal and c-terminal interactions. therefore, modulation of ntd function is considered an efficient strategy to target ar action. among various functional domains in different nuclear receptors, ntd is the least conserved and so could maybe become the best target site for ntd antagonists to achieve ar specificity. however the structural features of the ntd are undetermined due to a high degree of flexibility in its conformation. both biochemical and circular dichroism spectroscopy analysis suggest that ar ntd is highly disordered under native conditions, making it a difficult target for drug discovery. in 2008 there were reports of a chlorinated peptide, sintokamide a, isolated from marine sponges that effectively inhibits ar n-terminal domain-activated reporter gene transcription, see figure 8. the evidence presented was not sufficient to support the conclusion that sintokamide a directly inhibits the function of ar ntd, and the mechanism of action needs further investigation. selective androgen receptor modulators small molecule antiandrogens that are available today have undesirable side effects caused by complete, non-selective inhibition of ar action. to minimize these side effects, a new class of tissue selective androgen receptor modulators (sarms) has been proposed as a novel approach for the treatment of prostate cancer. these ligands should behave as antagonists in the prostate with either no activity or agonist activity in other target tissues, so as to have little or no effects in the anabolic tissues or central nervous system (cns). however discovering this new class of ligands might be challenging because the molecular mechanism of ar action is not well understood. several mechanisms have been proposed to achieve this tissue selectivity of ar ligands. the most definitive evidence exists for the role of 5-alpha reductase. 5-alpha reductase is only expressed in specific tissues and could therefore be a unique contributor to tissue selectivity. specific inhibition of the type 2 enzyme by finasteride blocks the conversion of testosterone to dht in the prostate. several approaches might make use of the potential tissue-specific conversion to develop sarms, including: inactive parent compounds that are activated by type 2 5-alpha reductase in the prostate to form antiandrogens. ar agonists that are inactivated by type 2 5-alpha reductase in the prostate. ar agonists that are converted to antiandrogens only by type 2 5-alpha reductase in the prostate. other small-molecule antiandrogens the development status of other small molecule antiandrogens undergoing research in 2011 can be seen in table 1. table 1 name of compoundstructurecompanystage of developmentother information ru58642roussel-uclaf sapreclinical – no further developments since 1998orally active and more potent than current small molecular antiandrogens. lg120907ligand pharmaceuticalspreclinicalorally active, strong antagonistic activity in the prostate without raising plasma levels of lh and testosterone. lg105ligand pharmaceuticalspreclinicalorally available, strong antagonistic activity in the prostate without raising plasma levels of lh and testosterone. seems to be more potent than lg120907. apalutamide (erleada)medivationapprovedhigh binding affinity to ar. unlike bicalutamide, it does not promote nuclear translocation and impairs both dna binding to androgen response elements and recruitment of coactivators. enzalutamide (xtandi)medivationapprovedhigh binding affinity to ar. unlike bicalutamide, it does not promote nuclear translocation and impairs both dna binding to androgen response elements and recruitment of coactivators. induces tumor cell apoptosis and has not agonist activity. bms-641988bristol-myers squibbphase i clinical – trial terminatedshowed increased potency compared to bicalutamide. phase i trial was discontinued because of an epileptic seizure in a patient. led to the findings that several antiandrogens produce off-target antagonist binding to gaba-a receptors. ch5137291chugai pharmaceutical co. ltd.preclinicalcompletely inhibits ar-mediated transactivation and proliferation of the crpc xenograft model lncap-bc2, which is bicalutamide-resistant. figure 9 ataric acidfigure 10 n-butylbenzenesulfonamide natural antiandrogens atraric acid and n-butylbenzenesulfonamide are natural compounds with antiandrogen properties which have been purified from the bark of the african tree pygeum africanum, see figures 9 and 10. in vitro assays have shown them both to be selective ar agonists and that they inhibit proliferation of several prostate cancer cell lines. atraric acid also hinders extracellular matrix invasion and both compounds are able to prevent androgen-induced nuclear translocation of the ar. more potent derivatives are currently being synthesized in hope of improving the pharmacological profile of these two compounds.