Comparison of the nonsteroidal antiandrogen (NSAA) bicalutamide with other antiandrogens reveals differences between the medications in terms of efficacy, tolerability, safety, and other parameters. Relative to the other first-generation NSAAs, flutamide and nilutamide, bicalutamide shows improved potency, efficacy, tolerability, and safety, and has largely replaced these medications in clinical practice. Compared to the second-generation NSAAs, enzalutamide and apalutamide, bicalutamide has inferior potency and efficacy but similar tolerability and safety and a lower propensity for drug interactions.
Relative to steroidal antiandrogens like cyproterone acetate and spironolactone, bicalutamide has better selectivity in its action, superior efficacy as an antagonist of the androgen receptor, and better tolerability. Bicalutamide also shows a better safety profile than cyproterone acetate. When used as a high-dosage monotherapy, bicalutamide shows slightly inferior effectiveness in the treatment of prostate cancer compared to castration and GnRH analogues but a different and potentially superior tolerability and safety profile. Unlike antigonadotropic antiandrogens like cyproterone acetate and GnRH analogues, bicalutamide does not suppress production of testosterone or estradiol and instead actually increases it, which has an important involvement in the differential side-effect profiles of the medications.
Overview Bicalutamide and the other nonsteroidal antiandrogens (NSAAs), since their introduction, have largely replaced cyproterone acetate (CPA), an older drug and steroidal antiandrogen (SAA), in the treatment of prostate cancer. Bicalutamide was the third to be marketed, with flutamide and nilutamide preceding, and followed by enzalutamide. Relative to the earlier antiandrogens, bicalutamide has substantially reduced toxicity, and in contrast to them, is said to have an excellent and favorable safety profile. For these reasons, as well as superior potency, tolerability, and pharmacokinetics, bicalutamide is preferred and has largely replaced flutamide and nilutamide in clinical practice. In accordance, bicalutamide is the most widely used antiandrogen in the treatment of prostate cancer. Between January 2007 and December 2009, it accounted in the for about 87.2% of prescriptions. Prior to the 2012 approval of enzalutamide, a newer and improved with greater potency and efficacy, bicalutamide was regarded as the standard-of-care antiandrogen in the treatment of the prostate cancer.
First-generation + Comparison of first-generation Property Flutamide Nilutamide Bicalutamide Half-life 5–6 hours ~2 days ~7 days 25% 20% 100% Dosage 250 mg 100 mg 150 mg Unique sideeffects/risks • Diarrhea• Hepatotoxicity• Photosensitivity • Nausea and vomiting• Visual disturbances• Alcohol intolerance• Interstitial pneumonitis • None Sources:
Flutamide and nilutamide are first-generation , similarly to bicalutamide, and all three drugs possess the same core mechanism of action of being selective antagonists. However, bicalutamide is the most potent of the three, with the highest affinity for the and the longest elimination half-life, and is the safest, least toxic, and best-tolerated. For these reasons, bicalutamide has largely replaced flutamide and nilutamide in clinical use, and is by far the most widely used first-generation .
Effectiveness In terms of binding to the , the active (R)-enantiomer of bicalutamide has 4-fold greater affinity relative to that of hydroxyflutamide, the active metabolite of flutamide (a prodrug), and 5-fold higher affinity relative to that of nilutamide. In addition, bicalutamide possesses the longest elimination half-life of the three drugs, with half-lives of 6–10 days for bicalutamide, 5–6 hours for flutamide and 8–9 hours for hydroxyflutamide, and 23–87 hours (mean 56 hours) for nilutamide. Due to the relatively short half-lives of flutamide and hydroxyflutamide, flutamide must be taken three times daily at 8-hour intervals, whereas bicalutamide and nilutamide may be taken once daily. For this reason, dosing of bicalutamide (and nilutamide) is more convenient than with flutamide. The greater affinity and longer elimination half-life of bicalutamide allow it to be used at relatively low dosages in comparison to flutamide (750–1500 mg/day) and nilutamide (150–300 mg/day) in the treatment of prostate cancer.
While it has not been directly compared to nilutamide, the effectiveness of bicalutamide has been found to be at least equivalent to that of flutamide in the treatment of prostate cancer in a direct head-to-head comparison. Moreover, indications of superior efficacy, including significantly greater relative decreases and increases in levels of prostate-specific antigen (PSA) and testosterone, respectively, were observed.
Tolerability and safety Comparative tolerability of Side effect Gynecomastia Breast pain Hot flashes Fatigue Nausea Diarrhea Constipation Back pain Visual disturbances Alcohol intolerance Hypertension Seizures Hepatotoxicity Key: : Not reported; : ≥ 1%, < 20%;: ≥ 20%, < 40%; : ≥ 40%. Sources:
The core side effects of such as gynecomastia, sexual dysfunction, and hot flashes occur at similar rates with the different drugs. Conversely, bicalutamide is associated with a significantly lower rate of diarrhea compared to flutamide. In fact, the incidence of diarrhea did not differ between the bicalutamide and placebo groups (6.3% vs. 6.4%, respectively) in the Early Prostate Cancer (EPC) clinical trial programme, whereas diarrhea occurs in up to 20% of patients treated with flutamide. The rate of nausea and vomiting appears to be lower with bicalutamide and flutamide than with nilutamide (approximately 30% incidence of nausea with nilutamide, usually rated as mild-to-moderate). In addition, bicalutamide (and flutamide) is not associated with alcohol intolerance, visual disturbances, or a high rate of interstitial pneumonitis. In terms of toxicity and rare reactions, as described above, bicalutamide appears to have the lowest relative risks of hepatotoxicity and interstitial pneumonitis, with respective incidences far below those of flutamide and nilutamide. In contrast to flutamide and nilutamide, no unique or specific complications have been linked to bicalutamide.
Second-generation Enzalutamide, along with the in-development apalutamide and darolutamide, are newer, second-generation . Similarly to bicalutamide and the other first-generation , they possess the same core mechanism of action of selective antagonism but are thought to bind to the androgen receptor with higher affinity, prevent nuclear translocation and DNA binding, and induce apoptosis without agonist activity. Theoretically such increased affinity may make them more efficacious. This is because cancer cells use different mechanisms to adapt and this increased affinity for the receptor make it more likely to bind to mutated receptors, to increased production of the receptors, and perhaps other mechanisms of resistance.
Effectiveness In comparison to bicalutamide, enzalutamide has 5- to 8-fold higher affinity for the , possesses mechanistic differences resulting in improved deactivation, shows increased (though by no means complete) resistance to mutations in prostate cancer cells causing a switch from antagonist to agonist activity, and has an even longer elimination half-life (8–9 days versus ~6 days for bicalutamide). In accordance, clinical findings suggest that enzalutamide is a significantly more potent and effective antiandrogen in comparison to first-generation NSAAs such as bicalutamide, flutamide, and nilutamide. Moreover, the medication has demonstrated greater clinical effectiveness in the treatment of prostate cancer in direct head-to-head comparisons with bicalutamide.
Tolerability and safety In terms of tolerability, enzalutamide and bicalutamide appear comparable in most regards, with a similar moderate negative effect on sexual function and activity for instance. However, enzalutamide has a risk of seizures and other central side effects such as anxiety and insomnia related to off-target GABAA receptor inhibition that bicalutamide does not appear to have. On the other hand, unlike with all of the earlier (flutamide, nilutamide, and bicalutamide), there has been no evidence of hepatotoxicity or elevated liver enzymes in association with enzalutamide treatment in clinical trials. In addition to differences in adverse effects, enzalutamide is a strong inducer of CYP3A4 and a moderate inducer of CYP2C9 and CYP2C19 and poses a high risk of major drug interactions (CYP3A4 alone being involved in the metabolism of approximately 50 to 60% of clinically important drugs), whereas drug interactions are few and minimal with bicalutamide.
Steroidal antiandrogens include cyproterone acetate (CPA), megestrol acetate, chlormadinone acetate, and spironolactone. These drugs are steroids, and similarly to , act as competitive antagonists of the , reducing androgenic activity in the body. In contrast to however, they are non-selective, also binding to other steroid hormone receptors, and exhibit a variety of other activities including progestogenic, antigonadotropic, glucocorticoid, and/or antimineralocorticoid. In addition, they are not silent antagonists of the , but are rather weak partial agonists with the capacity for both antiandrogenic and androgenic actions. Of the , is the only one that has been widely used in the treatment of prostate cancer. As antiandrogens, the have largely been replaced by the and are now rarely used in the treatment of prostate cancer, due to the superior selectivity, efficacy, and tolerability profiles of . However, some of them, namely and spironolactone, are still commonly used in the management of certain androgen-dependent conditions (e.g., acne and hirsutism in women) and as the antiandrogen component of feminizing hormone therapy for transgender women.
Effectiveness In a large-scale clinical trial that compared 750 mg/day flutamide and 250 mg/day monotherapies in the treatment of men with prostate cancer, the two drugs were found to have equivalent effectiveness on all endpoints. In addition, contrarily to the case of men, flutamide has been found in various clinical studies to be more effective than (and particularly spironolactone) in the treatment of androgen-dependent conditions such as acne and hirsutism in women. This difference in effectiveness in men and women may be related to the fact that like flutamide significantly increase androgen levels in men, which counteracts their antiandrogenic potency, but do not increase androgen levels in women. (In contrast to , , due to its progestogenic and hence antigonadotropic activity, does not increase and rather suppresses androgen levels in both sexes.)