The Development of the Bradykinin Agonist Labradimil as a Means to Increase the Permeability of the Blood-Brain Barrier

Labradimil (Cereport®; also formerly referred to as RMP-7) is a 9-amino-acid peptide designed for selectivity for the bradykinin B₂ receptor and a longer plasma half-life than bradykinin. It has been developed to increase the permeability of the blood-brain barrier (BBB) and is the first compound with selective bradykinin B₂ receptor agonist properties to progress from concept design through to tests of efficacy in patients.

In vitro studies demonstrate that labradimil has a longer half-life than bradykinin and selectively binds to bradykinin B₂ receptors, initiating typical bradykinin-like second messenger systems, including increases in intracellular calcium and phosphatidylinositol turnover. Initial proof of principle studies using electron microscopy demonstrated that intravenous labradimil increases the permeability of the BBB by disengaging the tight junctions of the endothelial cells that comprise the BBB. Autoradiographic studies in rat models further demonstrated that labradimil increases the permeability of the BBB in gliomas. Intravenous or intra-arterial labradimil increases the uptake of many different radiolabelled tracers and chemotherapeutic agents into the tumour in a dose-related fashion. These effects are selective for the tumour and for the brain surrounding the tumour, and are particularly robust in tumour areas that are normally relatively impermeable. The increased chemotherapeutic concentrations are maintained for at least 90 minutes, well beyond the transient effects on the BBB.

The increase in permeability with labradimil occurs rapidly but is transient, in that restoration of the BBB occurs very rapidly (2 to 5 minutes) following cessation of infusion. Even with continuous infusion of labradimil, spontaneous restoration of the barrier begins to occur within 10 to 20 minutes. Collectively, these data demonstrate that the B₂ receptor system that modulates permeability of the BBB is highly sensitive and autoregulated and that careful attention to the timing of labradimil and the chemotherapeutic agent is important to achieve maximal effects.

Survival studies in rodent models of both gliomas and metastatic tumours in the brain demonstrate that the enhanced uptake observed with the combination of labradimil and water-soluble chemotherapeutics enhances survival to a greater extent than achieved with chemotherapy alone. Finally, preliminary clinical trials in patients with gliomas provide confirmatory evidence that labradimil permeabilises the blood-brain tumour barrier and might, therefore, be used to increase delivery of agents such as carboplatin to tumours without the toxicity typically associated with dose escalation.