The Pharmacobiology of GnRH/LHRH Agonists and Antagonists Franklin D. Gaylis, MD, FACS Chairman, Evidence Based Medical Group of San Diego San Diego, California

Hypothalamus Pituitary Testes Prostate LHRH LH FSH GnRH Receptor Leydig Cells Sertoli Cells FSH Testosterone Other Target Tissues DHT Receptor Hormonal Influences on Prostate Cancer Brawer MK. Rev Urol. 2001;3:S1. Pending permission.

Evolution of Hormone Blockade < –03 GnRH Antagonists LHRH Agonist + Anti- androgen (CAB) LHRH Agonist DESOrchiectomy

NH 2 COOH Structure and Role of the GnRH Receptor

Examples of LHRH Agonists and Antagonists Agonists Leuprolide Leuprolide Goserelin Goserelin Buserelin BuserelinAntagonists Abarelix Abarelix Cetrorelix Cetrorelix Ganirelix Ganirelix Antide Antide Teverelix Teverelix ORG ORG A A-75998

PYROGLUHISTRPSERTYR GLY LEUARGPRO GLY HNHHNH LHRH Superagonists d-amino acid substitutions Adapted from Schally AV. Peptides. 1999;20:1247. With permission from Elsevier Science and AlphaMed Press.

LHRH Agonists: Mechanism of Action Bind to same site in the GnRH receptor as the naturally occurring LHRH Bind to same site in the GnRH receptor as the naturally occurring LHRH Stimulate production of LH and FSH initially Stimulate production of LH and FSH initially Eventually “exhaust” or internalize the GnRH receptor, thereby suppressing LH production Eventually “exhaust” or internalize the GnRH receptor, thereby suppressing LH production

PYROGLUHISTRP SER TYRGLY LEU ARG PROGLY HNHHNH GnRH Antagonists These residues replaced by d-amino acid in most antagonists Areas of key differences among antagonists

LHRH Agonists and Antagonists: Structural Differences LHRH Glu - His - Trp - Ser - Tyr -Gly - Leu - Arg - Pro - Gly Human Genome LHRH pGlu - His - Trp - Ser - Tyr - Gly - Leu - Arg - Pro - Gly - NH 2 Modified Agonist Leuprolide pGlu - His - Trp - Ser - Tyr - Dleu - Leu - Arg - Pro - NH - Et Superagonist Abarelix AcDNal-DClPhe-Dpal - Ser - NMTyr - DAsn - Leu - Ilys - Pro - DAla - NH 2 Antagonist Natural building blocks Synthetic building blocks

GnRH Antagonists: Mechanism of Action Bind to pituitary GnRH receptors, causing immediate suppression of LH and FSH Bind to pituitary GnRH receptors, causing immediate suppression of LH and FSH Turn off GnRH receptor by immediately and persistently blocking it, thus avoiding testosterone surge and “clinical flare” Turn off GnRH receptor by immediately and persistently blocking it, thus avoiding testosterone surge and “clinical flare”

Obstacles in the Development of the GnRH Antagonists Insufficient potency Insufficient potency Lack of solubility Lack of solubility Unacceptable histamine release Unacceptable histamine release Limitations with sustained release formulations Limitations with sustained release formulations

Abarelix First GnRH antagonist to progress through clinical trials for prostate cancer First GnRH antagonist to progress through clinical trials for prostate cancer Synthetic decapeptide GnRH antagonist that selectively binds to and immediately blocks the GnRH receptor Synthetic decapeptide GnRH antagonist that selectively binds to and immediately blocks the GnRH receptor Suppresses both LH and FSH secretion Suppresses both LH and FSH secretion Sustained-release formulation Sustained-release formulation

Agonists vs Antagonists: Clinical Effects Agonists Initial stimulation of LH production testosterone surge Initial stimulation of LH production testosterone surge Potential for testosterone surge and “clinical flare” Potential for testosterone surge and “clinical flare” May cause temporary rise in PSA levels May cause temporary rise in PSA levelsAntagonists Immediate, complete suppression of LH and testosterone Immediate, complete suppression of LH and testosterone PSA suppressed more quickly PSA suppressed more quickly No risk of testosterone surge No risk of testosterone surge

Median Testosterone Levels T Level (ng/dL) Leuprolide + bicalutamide Study Day Study Day Study Patients Through Day 85 Abarelix T Level (ng/dL) Adapted from Trachtenberg J, et al. J Urol. 2002;167:1670. With permission from Lippincott, Williams, & Wilkins.

Significance of FSH Suppression A preclinical study was conducted to characterize the expression of the follicle-stimulating hormone receptor (FSHR) in androgen-independent prostate cancer cell lines and human malignant prostate tissues. A preclinical study was conducted to characterize the expression of the follicle-stimulating hormone receptor (FSHR) in androgen-independent prostate cancer cell lines and human malignant prostate tissues. Results suggest that: Results suggest that: The FSHR expressed on these cancer cells is biologically active FSH can stimulate proliferation of androgen- resistant prostate cancer cells in the absence of androgen. FSH and/or its receptor may be potential targets in human prostate cancer. FSH and/or its receptor may be potential targets in human prostate cancer.

FSH Levels After GnRH Antagonist and After GnRH Agonist ± Anti-androgen GnRH antagonist (abarelix depot) GnRH agonist ± anti-androgen Median FSH Level (IU/L) Time (Days) Garnick MB, et al. Mol Urol. 2000;4:275.

Median Percentage Change from Baseline PSA Study Median Percentage Change in PSA n - LD n - AD Time (Days) Abarelix Leuprolide * * Note: Bars represent interquartile range. *P .001. Reprinted from McLeod D, et al. Urol. 2001;58:756. With permission from Elsevier Science.

Potential Applications for GnRH Antagonists Advanced disease where testosterone surge and clinical flare might be dangerous Advanced disease where testosterone surge and clinical flare might be dangerous Short periods of treatment, eg, downsizing prior to brachytherapy Short periods of treatment, eg, downsizing prior to brachytherapy Intermittent therapy Intermittent therapy Future research into the possible role of FSH and FSH receptors Future research into the possible role of FSH and FSH receptors

Summary Discovery of the decapeptide structure of LHRH in 1970s led to development of compounds that would effectively create androgen suppression, utilizing the LHRH structure Discovery of the decapeptide structure of LHRH in 1970s led to development of compounds that would effectively create androgen suppression, utilizing the LHRH structure GnRH receptor ultimately receives and mediates the primary stimulatory input to gonadotropes GnRH receptor ultimately receives and mediates the primary stimulatory input to gonadotropes

Summary LHRH agonists LHRH agonists –Initial treatment stimulates LH and FSH release Increased LH leads to testosterone surge and clinical flare Increased LH leads to testosterone surge and clinical flare –Chronic administration Causes downregulation of GnRH receptor and inhibition of LH Causes downregulation of GnRH receptor and inhibition of LH Does not suppress FSH Does not suppress FSH GnRH antagonists bind to, and persistently block, GnRH receptors GnRH antagonists bind to, and persistently block, GnRH receptors –Antagonists shut the receptor off –Immediately suppress LH and FSH production –Results in immediate reduction of testosterone to castrate levels –Avoid testosterone surge and “clinical flare”