Plenaxis (abarelix for injectable suspension), a gonadotropin-releasing hormone (GnRH) antagonist, lowers the male hormone testosterone in the blood stream. Testosterone makes some prostate cancers grow.

Plenaxis is approved for the treatment of advanced symptomatic prostate cancer, where LHRH agonist therapy is not appropriate and subjects have one or more of the following:

(1) Risk of neurological compromise due to metastases (2) Ureteral or bladder outlet obstruction due to local encroachment or metastatic disease (3) Severe bone pain from skeletal metastases persisting on narcotic analgesia.

A single-dose vial contains 113 mg of abarelix peptide in powder form, which must be mixed with .9% sodium chloride and injected intramuscularly. Plenaxis injections are taken every two weeks for the first month, and then every four weeks.

FDA approval of Plenaxis was based on one pivotal study enrolling 81 subjects with advanced symptomatic at risk for flare up if treated with LHRH agonists. The open-label, multicenter, uncontrolled, single-arm study was designed to see if patients could avoid orchiectomy through at least 12 weeks of treatment. Treatment was given for at least 6 months with the option to continue treatment in an extension trial. A dose of 100 mg of Plenaxis was administered via IM injection on Days 1, 15 and 29, then every 4 weeks.

Results showed that 10 of 13 subjects with bladder outlet obstruction and a bladder drainage catheter had had the catheter removed by 12 weeks of treatment. No subjects with vertebral or epidural metastases and without neurological symptoms developed neurological symptoms. In addition, 11 of 15 subjects with pain due to skeletal metastases were able to reduce the potency, dose and/or frequency of narcotic analgesia at 12 weeks. Medical castration, defined as serum total testosterone concentration =50 ng/dL),was achieved in 57 of the 72 patients (79%) by Day 8, and by 68 of 71 patients (96%) by Week 4.

Adverse events associated with the use of Plenaxis may include (but are not limited to) the following:

• 
  
• Hot flushes
• Sleep Disturbance
• Pain
• Myalgia
• Constipation
• Flushing
• Breast Enlargement
• Headache

Abarelix inhibits gonadotropin and related androgen production by directly and competitively blocking GnRH receptors in the pituitary gland. The drug directly suppresses luteinizing hormone (LH) and follicle stimulating hormone (FSH) secretion and thereby reducing the secretion of testosterone by the testes.

Crook JM, Szumacher E, Malone S, et al. Intermittent androgen suppression in the management of prostate cancer. Urology 1999;53:530-534.

Garnick MB, Gittelman M, Steidle C et al. Abarelix (PPI-149), a novel and potent GnRH antagonist, induces a rapid and profound prostate gland volume reduction (PGVR) and androgen suppression before brachytherapy (BT) or radiation therapy (XRT). J Urol 1998;159:220a.

Thompson IM, Zeidman EJ, Rodriguez FR. Sudden death due to disease flare with luteinizing hormone-releasing hormone agonist therapy for carcinoma of the prostate. J Urol 1990;144:1479-1470.

Trachtenberg J, Gittelman M, Steidle C, Barzell W, Friedel W, Pessis D, Fotheringham N, Campion M, Garnick MB. Abarelix-Depot versus Leuprolide Acetate plus bicalutamide for prostate cancer: results of multi-institutional, randomized, phase III study in 255 patients. Proc Am Soc Clin Oncol 2000:19:1307.

For additional information about Plenaxis or prostate cancer, please contact The Plenaxis Web Site