A pegylated growth hormone receptor antagonist, pegvisomant, does not enter the brain in humans

Background: GH receptors exist in the hippocampus, cerebral cortex, and hypothalamus, possibly influencing mood, cortical blood flow, and neuronal growth and mediating negative feedback. Rationale: Pegvisomant is a recombinant mutated GH molecule with high affinity, but little or no activating capability, for the GH receptor. It is used clinically as a GH antagonist. Hypothesis: Systemic pegvisomant enters brain interstitial fluid via putative choroid-plexus GH receptors, thereby allowing for antagonism of central actions of GH. Subjects and Location: Six adults requiring a cerebrospinal fluid (CSF) examination for nonneoplastic and noninflammatory syndromes participated at a tertiary medical center. Methods: Direct assays were conducted of serum and CSF pegvisomant concentrations 18-24 h after sc injection of pegvisomant (20 mg). Outcomes: Median (range) concentrations of pegvisomant in serum were 215 (74-539) μg/liter and in CSF 0.035 (0.010-0.28) μg/liter (P = 0.016). CSF drug levels were indistinguishable from assay threshold. Corresponding GH values were 0.29 (0.010-1.3) in serum and 0.075 μg/liter (0.01-0.13) in CSF. The geometric mean ratios of serum/CSF pegvisomant and GH concentrations were 5116:1 and 3.5:1, respectively, thus defining a more than 1400-fold difference between mutated and natural GH. Conclusions: Based upon CSF measurements, a pegylated GH-receptor antagonist does not cross the human blood-brain barrier, thereby sparing inhibition of central nervous system GH actions. Thus, the capability of this antagonist to stimulate GH secretion predominantly reflects its actions outside the blood-brain barrier, such as via the median eminence and/or via suppression of IGF-I concentrations.