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Pegvisomant: A growth hormonereceptor antagonist for the treatment of acromegaly
Growth Hormone & IGF Research 2000, Supplement B, Sll 9-S123
Review article
Pegvisomant: a growth hormone receptor antagonist for the treatment of acromegaly C. Parkinson and P, J. Trainer Departmentof Endocrinology,ChristieHospital,Manchester,UK
Summary Growth hormone receptor (GHR) dimerization is a prerequisite to the generation of growth hormone (GH) action. Pegvisomant is a GHR antagonist that has been designed to bind to the GHR at the cell surface and hence block this process. Initial studies suggest that pegvisomant is a highly effective antagonist of GH action in patients with acromegaly. The blockade of GH action, rather than the inhibition of pituitary GH secretion, represents a novel concept in the medical management of acromegaly. In this review, the design, efficacy, challenges and future role of pegvisomant are discussed.
© 2000 Harcourt Publishers Ltd
Key words: Acromegaly, growth hormone, growth hormone receptor antagonist, pegvisomant treatment.
INTRODUCTION Vigorous control of the growth hormone (GH)-insulinlike growth factor I (IGF-I) axis in acromegaly leads to a reduction in mortality and is a requirement of successful treatment >3. Medical treatment of acromegaly should reduce the mean serum GH level to below 5 mU/1, normalize serum IGF-I levels, avoid somatotroph tumour expansion and, ideally, produce a degree of tumour shrinkage 3. However, even when conventional treatments such as surgery, radiotherapy and medical therapy (dopamine agonists and somatostatin [SMS] analogues) are used in combination, it is still difficult to achieve tight biochemical control in a significant number of individuals. Existing treatment modalities for acromegaly modify GH release from somatotroph tumours, and their efficacy in reducing circulating IGF-I levels depends on individual tumour characteristics. For example, surgical outcome is critically dependent on the serum GH level at presentation and on tumour size. At best, remission Correspondenceto: P.J. Trainer,Departmentof Endocrinology,Christie Hospital,WilmslowRoad,Manchester,UK. Tel:+44 161 446 3664; Fax:+44 161 446 3772; E-mail:[email protected]
1096-6374/00/030119+05 $35.00/0
rates (based on post-operative mean serum GH levels of < 5 mU/1) for microadenomas approach 90%; however, results for macroadenomas remain disappointing, with approximately 50% of patients fating to achieve target serum GH levels following surgery 4-6. Similarly, the efficacy of SMS analogues is related to the number of functional SMS receptors produced by the tumour, and normalization of serum IGF-I is dependent on the pretreatment serum GH level r,8. Approximately 65% of patients with acromegaly receiving SMS analogues achieve serum IGF-I normalization, and the most potent dopamine agonist, cabergoline, reduces serum IGF-I levels to below 300 ~,g/1 in 39% of cases at doses up to 3.5 mg/week 9<4. In contrast to existing treatment modalities, pegvisomant represents a novel concept in the medical management of acromegaly. Rather than attempting to control the growth or secretory activity of the pituitary somatotroph turnout, pegvisomant binds to GH receptors (GHRs) on the cell surface, where it is able to block GH signal transduction and thereby GH action, including IGF-I generation. The efficacy of this GHR antagonist is therefore relatively independent of tumour characteristics.
© 2000 Harcourt Publishers Ltd
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C. Parkinson and P. J. Trainer
(A)
Dimerization
GHR
GHR IGFq
(B)
~ GHR
120R GHR
~
~
no IGF-I
Fig. 1 GH binding and the mechanism of action of the GHR antagonist. (A) Human GH has two distinct binding domains. Following initial binding of GH to the first GHR molecule at site 1, sequential binding at site 2 produces receptor dimerization, and signal transduction occurs. (B) A single amino acid substitution at position 120 of the GH molecule (G120R) completely disrupts binding at site 2. Reproduced with permission from Sensus Drug Development Corporation, Austin, Texas, USA.
GH S I G N A L T R A N S D U C T I O N
The GHR is a member of a large family of cytokine receptors of which the mechanism of signal transduction is hormone-induced receptor dimerization (see Groner et al. and Choi and Waxman, this supplement) 15.X-ray crystallography and alanine-scanning mutagenesis studies have revealed that the GH-GHR complex consists of a single GH molecule and two GHR molecules (Fig. 1). GH induces dimerization of two identical GHR molecules% GH is a 191-amino-acid peptide with a four-helix bundle core and has two disulphide bonds. Three discontinuous segments of the GH molecule make up binding site 1: the loop between residues 54 and 74, the C-terminal half of helix 4 and the N-terminal region of helix 1. Binding site 2 comprises the N-terminal residues of helices 1 and 3 lz. GHR binding occurs in a sequential manner. GH first binds to one GHR molecule via binding site 1, then binds to the second GHR molecule via binding site 2. Identical GHR molecules are bound at both sites 18-2°.
R A T I O N A L D E S I G N OF A G H R A N T A G O N I S T
The GH(GHR)2 complex is necessary for signal transduction and IGF-I generation, but a single point mutation at
position 120 of the GH molecule has been found to completely disrupt GHR binding at site 2 and thereby prevent GHR dimerization (Fig. 1). Transgenic mice expressing the gene encoding bovine GH that contained this site 2 mutation were of dwarf proportions 2~,22, The incorporation of eight amino acid substitutions into site 1 of a GHR antagonist, as in pegvisomant, was found to increase the affinity of the antagonist for GHR binding via site 1, compared with that of wild-type GH, resulting in the effective blockade of GH signal transduction (Fig. 2)23. The original unmodified GHR antagonist has a short plasma half-life (11 min) and therefore was effective only when administered continuously by subcutaneous
Fig. 2 Increased affinity of the GHR antagonist at binding site 1. The GHR antagonist (GHA) has eight amino acid substitutions within the first binding domain, leading to its increased affinity for binding to the GHR at site 1, compared with wild-type GH. One amino acid substitution at position 120 of the antagonist prevents GHR dimerization, so signal transduction cannot occur. Reproduced with permission from Sensus Drug Development Corporation, Austin, Texas, USA.
Pegvisomant: a GH receptor antagonist
121
infusion. Clark et •l. 24 demonstrated that the conjugation of 4,5-polyethylene glycol moieties (pegylation) to recombinant human GH produced a molecule possessing an extended plasma half-life and sustained efficacy. Similarly, pegylation of the antagonistic GH analogue increases its molecular size to 42-46 kDa, increases the half-life to 72 h (thereby allowing daily subcutaneous administration) and may lead to reduced antibody formation2L
Wilson 29 documented a 61% reduction in serum IGF-I and IGFBP-3 levels on day 3 of weekly pegvisomant treatment (1 mg/kg) in rhesus monkeys, compared with untreated controls. An approximately six-fold increase in serum GH levels (measured using a sandwich assay employing two monoclonal antibodies that do not crossreact with the antagonist) was observed following administration of the GHR antagonist.
MONITORING TREATMENT
CLINICAL STUDIES
Serum GH measurement cannot be used as a marker of disease activity in patients with acromegaly who are receiving pegvisomant. A fall in serum GH concentration is not anticipated following pegvisomant therapy, and serum GH may potentially rise due to reduced IGF-I feedback. There are also methodological problems in serum GH measurement in the presence of pegvisomant, because it has a high degree of structural homology with wild-type GH (which differs by just nine amino acids) and so is detected by standard GH radioimmunoassays, producing spuriously high serum GH values2! In the absence of serum GH as a marker of disease activity, the obvious candidate marker is serum IGF-I concentration. The use of serum IGF-I without concomitant serum GH information, however, is a new challenge. Widespread measurement of serum GH for over 20 years has allowed the relationship between serum GH level and mortality/morbidity to be clearly established. At present, the serum GH level is the best studied determinant of mortality in acromegaly. Retrospective studies have demonstrated the normalization of mortality risk when mean serum GH levels are reduced to less than 5 mU/p,2L Data on the relationship between serum IGF-I level and mortality are awaited, and to date only one publication has alluded to the normalization of mortality following the restoration of normal serum IGF-I levels in individuals with acromegaly2. In order to assist further investigation of this relationship, technical issues regarding the quality and comparability of serum IGF-I assays will need to be addressed, and standardized, age- and genderspecific reference ranges will need to be introduced 28. In patients receiving pegvisomant, useful insights into the severity of acromegaly may be obtained by the measurement not only of serum IGF-I but also of secondary markers of the GH-IGF-I axis, such as the GH-dependent IGF-binding protein-3 (IGFBP-3) and acid-labile subunit.
A phase I, placebo-controlled study in 36 healthy men receiving a single subcutaneous injection of pegvisomant at various doses (0.03, 0.1, 0.3 or 1.0 mg/kg) has recently been reported2! Peak plasma levels of pegvisomant occurred 3 6 h after drug administration, and a dosedependent suppression of serum IGF-I was observed, with approximately 50% reduction in serum IGF-I concentration on day 5 following the largest dose. A phase II, double-blind, placebo-controlled study provided proof of the concept that pegvisomant could lower serum IGF-I concentration in patients with active acromegaly. Individuals received a once-weekly subcutaneous injection of placebo or pegvisomant, 30-80 rag, for 6 weeks. A dose-related decline in serum IGF-I level and other markers of the GH-IGF-I axis was seen, but serum IGF-I level was normalized in only a small minority of the patients. The pharmacokinetics of the drug could possibly explain the failure of pegvisomant to normalize serum IGF-I concentration. As a reversible competitive receptor antagonist, sustained plasma concentrations of pegvisomant are crucial to its efficacy, and it was suspected that, with a half-life of approximately 72 h as well as a dosing and sampling interval of 1 week, adequate serum levels of the antagonist were not achieved in the days prior to sampling. In an open-labelled extension study of daily subcutaneous administration of pegvisomant (10-30 mg), serum IGF-I levels subsequently normalized in the vast majority of these patients 3°. A recent 12-week, phase III, multicentre, double-blind, randomized, placebo-controlled study of pegvisomant (10, 15 or 20mg/day) in 112 patients with active acromegaly has confirmed that pegvisomant is highly effective, well tolerated and not associated with significant side-effects31.
ANIMAL STUDIES
Because pegvisomant is not efficacious in rats, animal studies have concentrated on its actions in primates.
TUMOUR GROWTH
Pegvisomant does not attempt to control the growth or secretory activity of the somatotroph tumour. Theoretically, however, attenuated serum IGF-I feedback at the hypothalamic and pituitary level could promote somatotroph tumour expansion in individuals with
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acromegaly w h o are receiving pegvisomant. Encouragingly, especially as a n u m b e r of patients have b e e n receiving the drug for over 2 years, there is no evidence of t u m o u r enlargement to date. Despite this initial data, regular pituitary imaging will be required in patients with acromegaly receiving GHR antagonist therapy, until more experience with pegvisomant therapy is gained. This potential for t u m o u r expansion m a y be favourably altered by previous surgery or radiotherapy. ANTIBODY FORMATION A l t h o u g h pegylation should reduce antibody formation, pegvisomant is an alien protein and so is potentially antigenic, and antibodies to pegvisomant could reduce efficacy. The formation of antibody to r e c o m b i n a n t h u m a n GH is observed in 3% of GH-deficient children treated with GH, b u t antibody titres are low and do not inhibit g r o w t h 32,33. As with t u m o u r growth, the a c c u m u l a t e d experience of the last 2 years is encouraging, with no evidence of significant antibody formation.
COMBINATION THERAPY An attractive concept is combination medical therapy, using an agent that inhibits the release of s o m a t o t r o p h t u m o u r GH together with pegvisomant to block GH action at the target tissues. Significant t u m o u r shrinkage as a result of dopamine agonist therapy can be anticipated in t u m o u r s that co-secrete GH and prolactin, and a 25% reduction in t u m o u r v o l u m e has b e e n reported in a n u m b e r of patients receiving SMS analogues 34. To date, no studies have compared the efficacy of pegvisomant as single therapy to its combination with either a dopamine agonist or an SMS analogue. C o m b i n e d treatment m a y offer a synergistic effect, with serum IGF-I concentration being normalized at lower doses of pegvisomant.
CONCLUSIONS The ability to prevent GHR dimerization and IGF-I generation using pegvisomant represents a novel and exciting treatment option in acromegaly. Pegvisomant has b e e n rationally designed to out-compete wild-type GH for binding to GHR via site 1 and to disrupt site 2 binding completely, offering greater specificity of effect compared with existing medical treatments (e.g. somatostatin). Pegylation of the antagonist molecule allows daily subcutaneous administration and m a y reduce antibody formation. Initial data suggest that pegvisomant is a highly efficacious antagonist of GH action and offers the potential to achieve higher rates of serum IGF-I normalization t h a n is possible with current medical treatment modalities. If no
increase in t u m o u r size is observed and long-term studies confirm its efficacy, pegvisomant will be an attractive adjuvant therapy for controlling the GH-IGF-I axis in patients with acromegaly. REFERENCES 1. Bates AS, Van't Hoff W, Jones JM, Clayton RN. An audit of outcome of treatment in acromegaly. QJ Med 1993; 86: 293-299.
2. Swearingen B, Barker FG, Katznelson L et al. Long-term mortality after transsphenoidal surgery and adjunctive therapy for acromegaly. J Clin Endocrinol Metab 1998; 83: 3419-3426. 3. Melmed S. Tight control of growth hormone: an attainable outcome for acromegaly treatment. J Clin Endoctinol Metab 1998; 83: 3409-3410. 4. Sheaves R, Jenkins P, Blackburn Pet al. Outcome of transsphenoidal surgery for acromegaly using strict criteria for surgical cure. Clin Endocrinol (Oxf) 1996; 45:407-413. 5. Freda PU, Wardlaw SL, Post KD. Long-term endocrinological follow-up evaluation in 115 patients who underwent transsphenoidal surgery for acromegaly. J Neurosurg 1998; 89: 353-358. 6. Ahmed S, Elsheikh M, Stratton IM, Page RCL, Adams CBT,Wass JAH. Outcome of transsphenoidal surgery for acromegaly and its relationship to surgical experience. Clin Endocrinol (Ox0 1999; 50: 561-56Z Z Reubi JC, Landolt AM. The growth hormone responses to octreotide in acromegaly correlate with adenoma somatostatin receptor status. J Clin Endocrinol Metab 1989; 68: 844-850. 8. Newman CB, Melmed S, Snyder PJ et al. Safety and efficacy of long-term octreotide therapy of acromegaly: results of a multicenter trial in 103 patients-a clinical research center study. J Clin Endocrinol Metab 1995; 80: 2768-2775. 9. Lamberts SW, van der Lely AJ, de Herder WW, Hofland LJ. Octreotide. N EnglJ Med 1996; 334: 246-254. 10. Ezzat S, Redelmeier DA, Gnehm M, Harris AG. A prospective multicenter octreotide dose response study in the treatment of acromegaly. J Endocrinol Invest 1995; 18: 364-369. 11. Vance ML, Harris AG. Long-term treatment of 189 acromegalic patients with the somatostatin analogue octreotide. Results of the International Multicenter Acromegaly Study Group. Arch Intern Med 1991; 151: 1573-1578. 12. Flogstad AK, Halse J, Bakke Set al. Sandostatin JAR in acromegalic patients: long-term treatment. J Clin Endocrinol Metab 1997; 82: 23-28. 13. Lacranjan I, Atl6nson AB. The Sandostatin LAR Group. Results of a European multicentre study with Sandostatin LARin acromegalic patients. Pituitary 1999; 1: 105-114. 14. Abs R, Verhelst J, Maiter D et al. Cabergoline in the treatment of acromegaly: a study in 64 patients. J Clin Endocrinol Metab 1998; 83: 374-378. 15. Ullrich A, Schlessinger J. Signal transduction by receptors with tyrosine kinase activity. Cell 1990; 61 : 203-212. 16. Ultsch M, De Vos AM, Kossiakoff AA. Crystals of the complex between human growth hormone and the extracellular domain of its receptor, J Mol Biol 1991; 222: 865-868. 17. Cunningham BC,Jhurani I?,Wells JA. Receptor and antibody epitopes in human growth hormone identified by homologscanning mutagenesis. Science 1989; 243: 1330-1336. 18. Cunningham BC, Wells JA. High-resolution epitope mapping of hGH-receptor interactions by alanine-scanning mutagenesis. Science 1989; 244: 1081-1085. 19. Cunningham BC, Ultsch M, De Vos AM, Mulkerrin MG, Clauser
Pegvisomant: a GH receptor antagonist
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23.
24.
25.
26.
2Z
KR, Wells JA. Dimerization of the extracellular domain of the human growth hormone receptor by a single hormone molecule. Science 1991; 254: 821-825. De Vos AM, Ultsch M, Kossiakoff AA. Human growth hormone and extracellular domain of its receptor: crystal structure of the complex. Science 1992; 255: 306-312. Chen WY, Chen NY, Yun J e t al. Amino acid residues in the third alpha-helix of growth hormone involved in growth promoting activity. Mol Endocrinol 1995; 9: 292-302. Chen WY, Wight DC, Wagner TE, Kopchick JJ. Expression of a mutated bovine growth hormone gene suppresses growth of transgenic mice. Proc Natl Acad Sci USA 1990; 87: 5061-5065. Fuh G, Cunningham BC, Fukunaga R, Nagata S, Goeddel DV, Wells JA. Rational design of potent antagonists to the human growth hormone receptor. Science 1992; 256: 1677-1680. Clark R, Olson K, Fuh Get al. Long-acting growth hormones produced by conjugation with polyethylene glycol. J Biol Chem 1996; 271 : 21969-21977. Francis GE, Delgado C, Gisher D. Peg-modified proteins. In: Ahem TJ, Manning MC, eds. Stability of protein pharmaceuticals, part B: in v@o pathways of degradation and strategies for protein stabilization. New York (NY): Plenum Press, 1992; 235-285. Thorner MO, Strasburger CJ, Wu Z et al. Growth hormone (GH) receptor blockade with a PEG-modified GH (B2036-PEG) lowers serum insulin-like growth factor-I but does not acutely stimulate serum GH. J Clin Endocrinol Metab 1999; 84: 2098-2103. Orme SM, McNally RJQ, Cartwright R.A, Belchetz PE. Mortality and cancer incidence in acromegaly: a retrospective cohort study. J Clin Endocrinol Metab 1998; 83: 2730-2734.
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28. Melmed S. Confusion in clinical and laboratory GH and IGF-I reports. Pituitary 1999; 2: 171-172. 29. Wilson ME. Effects of estradiol and exogenous insulin-like growth factor I (IGF-I) on the IGF-I axis during growth hormone inhibition and antagonism. J Clin Endocrinol Metab 1998; 83: 4013-1421. 30. Barkan A, Dimaraki E, Besser GM et al. Treatment of acromegaly with B2036-PEG, a growth hormone receptor antagonist. The 81st Annual Meeting of the Endocrine Society; 1999 June 12-15; San Diego (CA). Bethesda (MD): The Endocrine Society. Oral Presentation No. OR14-6:82 (Abstract). 31. Trainer PJ, Besser GM, Klibanski A, Ereda PU, Melmed S. A phase III study of B2036-PEG, a growth hormone antagonist, in the treatment of acromegaly. The 81st Annual Meeting of the Endocrine Society; 1999 June 12-15; San Diego (CA). Bethesda (MD): The Endocrine Society. Poster Presentation No. P1-46: 144-145 (Abstract). 32. Zeisel HJ, Lutz A, von Petrykowski W. Immunogenicity of a mammalian cell-derived recombinant human growth hormone preparation during long-term treatment. Horm Res 1992; 37(Suppl 2): 47-55. 33. Massa G, Vanderschueren-Lodeweyckx M, Bouillon R. Five-year follow-up of growth hormone antibodies in growth hormone deficient children treated with recombinant human growth hormone. Clin Endocrinol (Oxf) 1993; 38: 137-142. 34. Newman CB, Melmed S, George A et al. Octreotide as primary therapy for acromegaly. J Clin Endocrinol Metab 1998; 83: 3034-3040.
Review article
Pegvisomant: a growth hormone receptor antagonist for the treatment of acromegaly C. Parkinson and P, J. Trainer Departmentof Endocrinology,ChristieHospital,Manchester,UK
Summary Growth hormone receptor (GHR) dimerization is a prerequisite to the generation of growth hormone (GH) action. Pegvisomant is a GHR antagonist that has been designed to bind to the GHR at the cell surface and hence block this process. Initial studies suggest that pegvisomant is a highly effective antagonist of GH action in patients with acromegaly. The blockade of GH action, rather than the inhibition of pituitary GH secretion, represents a novel concept in the medical management of acromegaly. In this review, the design, efficacy, challenges and future role of pegvisomant are discussed.
© 2000 Harcourt Publishers Ltd
Key words: Acromegaly, growth hormone, growth hormone receptor antagonist, pegvisomant treatment.
INTRODUCTION Vigorous control of the growth hormone (GH)-insulinlike growth factor I (IGF-I) axis in acromegaly leads to a reduction in mortality and is a requirement of successful treatment >3. Medical treatment of acromegaly should reduce the mean serum GH level to below 5 mU/1, normalize serum IGF-I levels, avoid somatotroph tumour expansion and, ideally, produce a degree of tumour shrinkage 3. However, even when conventional treatments such as surgery, radiotherapy and medical therapy (dopamine agonists and somatostatin [SMS] analogues) are used in combination, it is still difficult to achieve tight biochemical control in a significant number of individuals. Existing treatment modalities for acromegaly modify GH release from somatotroph tumours, and their efficacy in reducing circulating IGF-I levels depends on individual tumour characteristics. For example, surgical outcome is critically dependent on the serum GH level at presentation and on tumour size. At best, remission Correspondenceto: P.J. Trainer,Departmentof Endocrinology,Christie Hospital,WilmslowRoad,Manchester,UK. Tel:+44 161 446 3664; Fax:+44 161 446 3772; E-mail:[email protected]
1096-6374/00/030119+05 $35.00/0
rates (based on post-operative mean serum GH levels of < 5 mU/1) for microadenomas approach 90%; however, results for macroadenomas remain disappointing, with approximately 50% of patients fating to achieve target serum GH levels following surgery 4-6. Similarly, the efficacy of SMS analogues is related to the number of functional SMS receptors produced by the tumour, and normalization of serum IGF-I is dependent on the pretreatment serum GH level r,8. Approximately 65% of patients with acromegaly receiving SMS analogues achieve serum IGF-I normalization, and the most potent dopamine agonist, cabergoline, reduces serum IGF-I levels to below 300 ~,g/1 in 39% of cases at doses up to 3.5 mg/week 9<4. In contrast to existing treatment modalities, pegvisomant represents a novel concept in the medical management of acromegaly. Rather than attempting to control the growth or secretory activity of the pituitary somatotroph turnout, pegvisomant binds to GH receptors (GHRs) on the cell surface, where it is able to block GH signal transduction and thereby GH action, including IGF-I generation. The efficacy of this GHR antagonist is therefore relatively independent of tumour characteristics.
© 2000 Harcourt Publishers Ltd
120
C. Parkinson and P. J. Trainer
(A)
Dimerization
GHR
GHR IGFq
(B)
~ GHR
120R GHR
~
~
no IGF-I
Fig. 1 GH binding and the mechanism of action of the GHR antagonist. (A) Human GH has two distinct binding domains. Following initial binding of GH to the first GHR molecule at site 1, sequential binding at site 2 produces receptor dimerization, and signal transduction occurs. (B) A single amino acid substitution at position 120 of the GH molecule (G120R) completely disrupts binding at site 2. Reproduced with permission from Sensus Drug Development Corporation, Austin, Texas, USA.
GH S I G N A L T R A N S D U C T I O N
The GHR is a member of a large family of cytokine receptors of which the mechanism of signal transduction is hormone-induced receptor dimerization (see Groner et al. and Choi and Waxman, this supplement) 15.X-ray crystallography and alanine-scanning mutagenesis studies have revealed that the GH-GHR complex consists of a single GH molecule and two GHR molecules (Fig. 1). GH induces dimerization of two identical GHR molecules% GH is a 191-amino-acid peptide with a four-helix bundle core and has two disulphide bonds. Three discontinuous segments of the GH molecule make up binding site 1: the loop between residues 54 and 74, the C-terminal half of helix 4 and the N-terminal region of helix 1. Binding site 2 comprises the N-terminal residues of helices 1 and 3 lz. GHR binding occurs in a sequential manner. GH first binds to one GHR molecule via binding site 1, then binds to the second GHR molecule via binding site 2. Identical GHR molecules are bound at both sites 18-2°.
R A T I O N A L D E S I G N OF A G H R A N T A G O N I S T
The GH(GHR)2 complex is necessary for signal transduction and IGF-I generation, but a single point mutation at
position 120 of the GH molecule has been found to completely disrupt GHR binding at site 2 and thereby prevent GHR dimerization (Fig. 1). Transgenic mice expressing the gene encoding bovine GH that contained this site 2 mutation were of dwarf proportions 2~,22, The incorporation of eight amino acid substitutions into site 1 of a GHR antagonist, as in pegvisomant, was found to increase the affinity of the antagonist for GHR binding via site 1, compared with that of wild-type GH, resulting in the effective blockade of GH signal transduction (Fig. 2)23. The original unmodified GHR antagonist has a short plasma half-life (11 min) and therefore was effective only when administered continuously by subcutaneous
Fig. 2 Increased affinity of the GHR antagonist at binding site 1. The GHR antagonist (GHA) has eight amino acid substitutions within the first binding domain, leading to its increased affinity for binding to the GHR at site 1, compared with wild-type GH. One amino acid substitution at position 120 of the antagonist prevents GHR dimerization, so signal transduction cannot occur. Reproduced with permission from Sensus Drug Development Corporation, Austin, Texas, USA.
Pegvisomant: a GH receptor antagonist
121
infusion. Clark et •l. 24 demonstrated that the conjugation of 4,5-polyethylene glycol moieties (pegylation) to recombinant human GH produced a molecule possessing an extended plasma half-life and sustained efficacy. Similarly, pegylation of the antagonistic GH analogue increases its molecular size to 42-46 kDa, increases the half-life to 72 h (thereby allowing daily subcutaneous administration) and may lead to reduced antibody formation2L
Wilson 29 documented a 61% reduction in serum IGF-I and IGFBP-3 levels on day 3 of weekly pegvisomant treatment (1 mg/kg) in rhesus monkeys, compared with untreated controls. An approximately six-fold increase in serum GH levels (measured using a sandwich assay employing two monoclonal antibodies that do not crossreact with the antagonist) was observed following administration of the GHR antagonist.
MONITORING TREATMENT
CLINICAL STUDIES
Serum GH measurement cannot be used as a marker of disease activity in patients with acromegaly who are receiving pegvisomant. A fall in serum GH concentration is not anticipated following pegvisomant therapy, and serum GH may potentially rise due to reduced IGF-I feedback. There are also methodological problems in serum GH measurement in the presence of pegvisomant, because it has a high degree of structural homology with wild-type GH (which differs by just nine amino acids) and so is detected by standard GH radioimmunoassays, producing spuriously high serum GH values2! In the absence of serum GH as a marker of disease activity, the obvious candidate marker is serum IGF-I concentration. The use of serum IGF-I without concomitant serum GH information, however, is a new challenge. Widespread measurement of serum GH for over 20 years has allowed the relationship between serum GH level and mortality/morbidity to be clearly established. At present, the serum GH level is the best studied determinant of mortality in acromegaly. Retrospective studies have demonstrated the normalization of mortality risk when mean serum GH levels are reduced to less than 5 mU/p,2L Data on the relationship between serum IGF-I level and mortality are awaited, and to date only one publication has alluded to the normalization of mortality following the restoration of normal serum IGF-I levels in individuals with acromegaly2. In order to assist further investigation of this relationship, technical issues regarding the quality and comparability of serum IGF-I assays will need to be addressed, and standardized, age- and genderspecific reference ranges will need to be introduced 28. In patients receiving pegvisomant, useful insights into the severity of acromegaly may be obtained by the measurement not only of serum IGF-I but also of secondary markers of the GH-IGF-I axis, such as the GH-dependent IGF-binding protein-3 (IGFBP-3) and acid-labile subunit.
A phase I, placebo-controlled study in 36 healthy men receiving a single subcutaneous injection of pegvisomant at various doses (0.03, 0.1, 0.3 or 1.0 mg/kg) has recently been reported2! Peak plasma levels of pegvisomant occurred 3 6 h after drug administration, and a dosedependent suppression of serum IGF-I was observed, with approximately 50% reduction in serum IGF-I concentration on day 5 following the largest dose. A phase II, double-blind, placebo-controlled study provided proof of the concept that pegvisomant could lower serum IGF-I concentration in patients with active acromegaly. Individuals received a once-weekly subcutaneous injection of placebo or pegvisomant, 30-80 rag, for 6 weeks. A dose-related decline in serum IGF-I level and other markers of the GH-IGF-I axis was seen, but serum IGF-I level was normalized in only a small minority of the patients. The pharmacokinetics of the drug could possibly explain the failure of pegvisomant to normalize serum IGF-I concentration. As a reversible competitive receptor antagonist, sustained plasma concentrations of pegvisomant are crucial to its efficacy, and it was suspected that, with a half-life of approximately 72 h as well as a dosing and sampling interval of 1 week, adequate serum levels of the antagonist were not achieved in the days prior to sampling. In an open-labelled extension study of daily subcutaneous administration of pegvisomant (10-30 mg), serum IGF-I levels subsequently normalized in the vast majority of these patients 3°. A recent 12-week, phase III, multicentre, double-blind, randomized, placebo-controlled study of pegvisomant (10, 15 or 20mg/day) in 112 patients with active acromegaly has confirmed that pegvisomant is highly effective, well tolerated and not associated with significant side-effects31.
ANIMAL STUDIES
Because pegvisomant is not efficacious in rats, animal studies have concentrated on its actions in primates.
TUMOUR GROWTH
Pegvisomant does not attempt to control the growth or secretory activity of the somatotroph tumour. Theoretically, however, attenuated serum IGF-I feedback at the hypothalamic and pituitary level could promote somatotroph tumour expansion in individuals with
122
C. Parkinson and P. J. Trainer
acromegaly w h o are receiving pegvisomant. Encouragingly, especially as a n u m b e r of patients have b e e n receiving the drug for over 2 years, there is no evidence of t u m o u r enlargement to date. Despite this initial data, regular pituitary imaging will be required in patients with acromegaly receiving GHR antagonist therapy, until more experience with pegvisomant therapy is gained. This potential for t u m o u r expansion m a y be favourably altered by previous surgery or radiotherapy. ANTIBODY FORMATION A l t h o u g h pegylation should reduce antibody formation, pegvisomant is an alien protein and so is potentially antigenic, and antibodies to pegvisomant could reduce efficacy. The formation of antibody to r e c o m b i n a n t h u m a n GH is observed in 3% of GH-deficient children treated with GH, b u t antibody titres are low and do not inhibit g r o w t h 32,33. As with t u m o u r growth, the a c c u m u l a t e d experience of the last 2 years is encouraging, with no evidence of significant antibody formation.
COMBINATION THERAPY An attractive concept is combination medical therapy, using an agent that inhibits the release of s o m a t o t r o p h t u m o u r GH together with pegvisomant to block GH action at the target tissues. Significant t u m o u r shrinkage as a result of dopamine agonist therapy can be anticipated in t u m o u r s that co-secrete GH and prolactin, and a 25% reduction in t u m o u r v o l u m e has b e e n reported in a n u m b e r of patients receiving SMS analogues 34. To date, no studies have compared the efficacy of pegvisomant as single therapy to its combination with either a dopamine agonist or an SMS analogue. C o m b i n e d treatment m a y offer a synergistic effect, with serum IGF-I concentration being normalized at lower doses of pegvisomant.
CONCLUSIONS The ability to prevent GHR dimerization and IGF-I generation using pegvisomant represents a novel and exciting treatment option in acromegaly. Pegvisomant has b e e n rationally designed to out-compete wild-type GH for binding to GHR via site 1 and to disrupt site 2 binding completely, offering greater specificity of effect compared with existing medical treatments (e.g. somatostatin). Pegylation of the antagonist molecule allows daily subcutaneous administration and m a y reduce antibody formation. Initial data suggest that pegvisomant is a highly efficacious antagonist of GH action and offers the potential to achieve higher rates of serum IGF-I normalization t h a n is possible with current medical treatment modalities. If no
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