Antagonists of growth hormone-releasing hormone cross the blood-brain barrier: A potential applicability to treatment of brain tumors

Hypothalamic growth hormone (GH)-releasing hormone (GHRH) stimulates the synthesis and release of GH from the pituitary gland. GHRH and its mRNA are also found in human cancers of the breast ovary, prostate, lung, and other tumors, suggesting that GHRH is also a tumor growth factor. Various studies show that GHRH antagonists have antiproliferative effects in many tumor models; however, glioblastomas were examined only recently. Previous studies have demonstrated that s.c. administration of GHRH antagonist (JV-1-36) inhibited growth of s.c. U-87MG human glioblastomas and increased survival of nude mice with orthotopic implants of glioblastomas. Although treatment with JV-1-36 reduced tumorigenicity, it is not known whether peripherally administered GHRH antagonists can cross the blood-brain barrier. Brain endothelial cells joined by tight junctions form the blood-brain barrier, a "barrier" between the general circulation and the CNS. In this study, we administered a GHRH antagonist (JV-1-42) and showed that, after i.v. injection, iodinated JV-1-42 (¹³¹I-JV-1-42) enters the brain intact at a rate of 0.8514 μl/g per min with a serum half-life of 12.2 min. A one-site binding hyperbolic model indicated that the maximal percent of i.v. dose taken up per gram of brain was 0.41%. Coinjection of unlabeled JV-1-42 indicated that the transport from blood to brain is not saturable; however, transport from brain to blood is saturable and involves P-glycoprotein. Taken together, these results demonstrate that i.v.-administered ¹³¹I-JV-1-42 readily crosses the blood-brain barrier and accumulates in the brain. This finding indicates that GHRH antagonists could provide a potential treatment for malignant glioblastomas.