Enzalutamide | Androgen Receptor Inhibitor | Treatment for Prostate Cancer | Cancer Drug

Enzalutamide [4-(3-(4-Cyano-3-(trifluoromethyl)phenyl)-5,5-dimethyl-4-oxo-2-thioxoimidazolidin-1-yl)-2-fluoro-N-methylbenzamide] is an orally available, small-molecule androgen receptor (AR) inhibitor that is indicated for the treatment of metastatic, castration-resistant, prostate cancer (mCRPC) which has progressed despite treatment with Docetaxel.  Its is a non-steroidal anti-androgen belonging to the chemical class of thiohydantoin [1, 2].

Enzalutamide: 2D and 3D Structure

It is the results from a randomized, double-blind, placebo-controlled, multinational, phase III trial in patients with mCRPC progressing after docetaxel therapy, which proved:

(a) Enzalutamide significantly prolonged overall survival (OS);

(b) Enzalutamide delayed prostate specific antigen progression; and

(c) Enzalutamide prolonged radiographic progression-free survival; and

(d) Enzalutamide prolonged time to the first skeletal event.

The median OS was 18.4 months in the Enzalutamide group and 13.6 months in the placebo group, which represents a 37 % reduction in the mortality risk in the Enzalutamide group.

Enzalutamide is proved to an efficacious and well tolerated treatment for this severe, rapidly progressive disease and also is associated with significant benefits in health-related quality of life and in pain palliation (def: patients with terminal diseases usually need palliation. It's a kind of care that makes you feel better, even though it can't cure you).

Prostate Cancer to Castration-resistant Prostate Cancer and Androgen Recpetor

Prostate cancer (PC) is the most common cancer and among the three leading causes of cancer deaths in men in the United States and in Europe. The activation of androgen receptor (AR) signaling is crucial for PC growth at all stages of the disease.

Although prostate cancer can be initially treated with either castration or with the first-generation androgen receptor (AR) antagonists such as bicalutamide, nilutamide, and flutamide (which is oxidized to the active metabolite hydroxyflutamide), after a period of approximately 2-4 years, the cancer becomes resistant to AR treatment. Indeed in this castration resistant stage (formerly called hormone refractory or “androgen-independent”), former AR antagonists such as bicalutamide become partial agonists and their use in cancer treatment must be discontinued. Most PC patients eventually build up resistance to the treatments and develop a more aggressive form of the disease called castration-resistant prostate cancer (CRPC) that is associated with tumor progression and poor prognosis [3].

Castration-resistant prostate cancer (CRPC) typically arises through mechanisms involving AR, as shown by studies demonstrating the role of autocrine synthesis of androgens and AR protein overexpression in CRPC. AR aberrations commonly associated with CRPC include AR amplifications, mutations, and constitutively active AR splice variants. Amplification of the AR gene leading to AR protein overexpression and mutations in the ligand binding domain (LBD) of AR can make the receptor more sensitive to growth-stimulating effects of low androgen concentrations and turn antagonist responses to agonistic.

For years, Docetaxel was the only treatment for CRPC showing a median prolongation of survival of 2.9 months. In recent years, however, new treatment options for CRPC with different mechanisms of action have become available. Many of the new treatments target AR signaling such as CYP17A1 inhibitor abiraterone acetate and a second-generation AR antagonist enzalutamide.

Certain AR mutations have been linked to the development of resistance to specific antiandrogens, e.g. W741L and T877A mutations have been shown to mediate resistance to first-generation anti-androgens bicalutamide and hydroxyflutamide, respectively. However, despite the initial response to second-generation AR-targeted agents, resistance develops in nearly all men with metastatic CRPC. Recently, a F876L missense mutation in the LBD of the AR was identified to confer resistance to Enzalutamide and ARN-509, an AR antagonist presently in a phase 3 study, by switching these antagonists to agonists [3].

Enzalutamide as Androgen Receptor Inhibitor

In a competition assay using 16β-[¹⁸F]fluoro-5α-DHT (18-FDHT) to measure relative AR binding affinity, Enzalutamide bound AR in castration-resistant LNCaP/AR human prostate cancer cells (engineered to express higher levels of wild-type AR to mimic the clinical scenario) with 5-8 fold greater affinity than Bicalutamide (IC₅₀ Enzalutamide, Bicalutamide = 36, 159 nM, respectively) and only 2-3 fold reduced affinity relative to the derivative of the native ligand FDHT.

In competitive AR binding assays, the inhibition constant (K_i) value for Enzalutamide was 86 nM. Moreover, it was potent and full antagonists for human AR (hAR) with IC₅₀ values 219 nM as shown by transactivation assays in AR-HEK293 cells stably expressing full-length hAR and an androgen-responsive luciferase reporter gene construct.

Emergence of mutations in AR has been suggested to drive resistance to antiandrogen therapies. The effects of antiandrogens on mutant AR(F876L), AR(W741L), and AR(T877A) were studied in transactivation assays in human U2-OS osteosarcoma cells transiently transfected with expression vectors encoding the corresponding mutant AR and an androgen-responsive luciferase reporter gene construct. The F876L substitution in AR switched enzalutamide from antagonists to agonists. The IC₅₀ values were found to for wtAR, AR(W741L), and AR(T877A) as 155 nM, greater than 10 uM and 296 nM, respectively [3].

In vitro, the active metabolite N-desmethyl enzalutamide has similar activity to parent Enzalutamide.

N-desmethyl enzalutamide

Reports suggest that Enzalutamide inhibits the AR signalling pathway by competitively inhibiting receptor binding by dihydrotestosterone (DHT). It binds strongly to ARs, with an affinity that is higher than that of earlier generation anti-androgens. Moreover it is active in the presence of AR amplification and overexpression. In comparison to first generation inhibitors, Enzalutamide also inhibits DHT-AR nuclear translocation, directly interfering with AR-mediated transcription. The result is decreased proliferation of prostate cancer cells and increased cell death.

Still, Enzalutamide fails at few check points:

(a) Resistance due to emergence of mutations in AR, namely AR(F876L), which switched enzalutamide from antagonists to agonists.

(b) AR splice variant isoforms, which remain active despite missing the androgen ligand-binding domain, occur frequently in metastasis samples from patients with mCRPC, promote tumour cell growth, and are associated with a poor prognosis. mCRPC cell lines expressing AR variants showed robust growth in the presence of bicalutamide and enzalutamide, despite antagonism of full length AR in these cells by both drugs.

(c) In prostate cancer, the nuclear factor kappa-B 2 (NFKB2)/p52 transcription pathway is involved in aberrant AR activation. Results suggest that the NFKB2/p52 transcription pathway may be important in the development of resistance to Enzalutamide.

Dosages and Approvals:

Enzalutamide (Tradename: Xtandi) is approved in patients with mCRPC that has progressed following Docetaxel therapy. The recommended Enzalutamide dosage is 160 mg once daily, administered in four 40 mg capsules. The capsules must be undissolved and swallowed whole, and can be taken with or without food.

In patients who develop an intolerable side effect or any toxicity, Enzalutamide should be withheld for one week, or until toxic symptoms improve, at which time treatment can be resumed at the original dosage or at a reduced dosage of 80 or 120 mg once daily, if warranted.

The agent was developed by synthetic and medicinal chemist Michael E. Jung of the University of California, Los Angeles, and coworkers, in collaboration with oncologist Charles L. Sawyers’ group at Memorial Sloan-Kettering Cancer Center (MSKCC), in New York City. Later they licensed their patents to Medivation, in San Francisco. Based upon the findings of various clinical trials; on August 2012, the United States (U.S.) Food and Drug Administration (FDA) approved enzalutamide for the treatment of castration-resistant prostate cancer [4].

Summary

Common name: MDV3100; MDV 3100; MDV-3100

Trademarks: Xtandi

Molecular Formula: C21H16F4N4O2S

CAS Registry Number: 915087-33-1

CAS Name: 4-(3-(4-Cyano-3-(trifluoromethyl)phenyl)-5,5-dimethyl-4-oxo-2-thioxoimidazolidin-1-yl)-2-fluoro-N-methylbenzamide

Molecular Weight: 464.44

SMILES:N#Cc1c(C(F)(F)F)cc(N2C(C(C)(C)N(c3ccc(C(N(C)[H])=O)c(F)c3)C2=S)=O)cc1

InChI Key: WXCXUHSOUPDCQV-UHFFFAOYSA-N

InChI:InChI=1S/C21H16F4N4O2S/c1-20(2)18(31)28(12-5-4-11(10-26)15(8-12)21(23,24)25)19(32)29(20)13-6-7-14(16(22)9-13)17(30)27-3/h4-9H,1-3H3,(H,27,30)

Mechanism of Action: Androgen Receptor (AR) Inhibitor

Activity: Treatment of Metastatic, Castration-resistant, Prostate Cancer; Treatment of mCRPC; Anti-cancer Agents

Status: Launched 2012 (US)

Chemical Class: Thio-hydantoins; Small-molecules; Sulfur containing; Flourine containing; Nitrile containing

Originator: Medivation Pharmaceuticals

Enzalutamide Synthesis

WO2011106570A1: The patent reports the best possible synthetic route to prepare Enzalutamide. It is free from any tedious separation step (as compared to earlier reported methods) and is economical too. It appears to be the industrial process.

Intermediate 1:

Intermediate 2:

Final Synthesis:

J Med Chem 2010 53(7), 2779-2796: It is one of the earliest reported synthesis procedure.

Identifications:

1H NMR (Estimated) for Enzalutamide

Experimental: ¹H NMR (CDCl₃, 400 MHz) δ 1.61 (s, 6H), 3.07 (d, 3H, J=4.1 Hz), 6.71 (m, 1 H), 7.15 (dd, 1H, J=11.7, 2.0 Hz), 7.24 (dd, 1H, J=8.4, 2.0 Hz), 7.83 (dd, 1H, J=8.2, 2.1 Hz), 7.95 (d, 1H, J=2.1 Hz), 7.99 (d, 1H, J=8.2 Hz), 8.28 (dd, 1H, J = 8.4, 8.4 Hz).

13C NMR (Estimated) for Enzalutamide

Experimental: ¹³C NMR (CDCl₃, 125 MHz) δ 23.8, 26.9, 66.5, 110.3, 114.6, 117.7, 117.9, 121.7 (q, J= 272.3 Hz), 126.1, 126.9 (q, J = 4.6 Hz), 132.0, 133.3, 133.6 (q, J=33.4 Hz), 135.2, 136.7, 138.9 (d, J=10.8 Hz), 160.3 (d, J= 248.6 Hz), 162.6 (d, J=3.3 Hz), 174.3, 179.6.

Sideeffects:

Fatigue was the most common adverse event; it occurred at a grade 3 or 4 level of severity in 6 and 7% of Enzalutamide and placebo recipients, respectively. Other adverse events occurring in greater than 10% of enzalutamide recipients and in more than 2% more enzalutamide than placebo recipients are diarrhoea, hotflush, musculo-skeletal pain and headache.

Enzalutamide carries a small increased risk of seizures that appears to be dose-dependent.

Moreover, other clinically-relevant adverse events in Enzalutamide and placebo recipients were, grade 1-4 haematuria (7 and 5%), cardiac disorders (6 and 8%), hypertension (7 and 3%) and myocardial infarction (less than 1%). Regarding laboratory abnormalities, in the Enzalutamide and placebo groups, neutropenia (15 and 6%), elevations in alanine aminotransferase (ALT, 10 and 18%) and grade 1-4 hyperbilirubinaemia (3 and 2%) were observed. There were neither any clinically relevant changes associated with Enzalutamide treatment in the corrected QT interval/electrocardiographic data nor any laboratory disturbances suggesting the development of metabolic syndrome with Enzalutamide

References:
1. Sanford, M. Enzalutamide: A Review of Its Use in Metastatic, Castration-Resistant Prostate Cancer. Drugs 2013, 73(15), 1723-1732. (FMO only)
2. Jung, M. E.; et. al. Structure-Activity Relationship for Thiohydantoin Androgen Receptor Antagonists for Castration-Resistant Prostate Cancer (CRPC). J Med Chem 2010, 53(7), 2779-2796. (FMO only)
3. Moilanen, A. M.; et. al. Discovery of ODM-201, a new-generation androgen receptor inhibitor targeting resistance mechanisms to androgen signaling-directed prostate cancer therapies. Sci Rep 2015, 3(5), 12007. (free copy)
4. Borman, S. New Prostate Cancer Agent Class. Chemical and Engineering News 2008, 86(38), 84-87. (FMO only)
5. Greenfield, S.; et. al. Processes for the synthesis of diarylthiohydantoin and diarylhydantoin compounds. WO2011106570A1