Arena Pharmaceuticals, Inc. and Eisai Inc. announced the completion of a clinical study that measured lorcaserin concentrations in human Cerebrospinal Fluid (CSF) and plasma and related data analyses. The study was conducted to provide additional data that may be informative for determining the human relevance of the observation of brain astrocytoma in male rats.

Using the results of this study and other preclinical and clinical studies, Arena estimates that the mean exposure of the human brain to lorcaserin at the clinically tested dose (10 mg dosed twice daily (BID)) is approximately 1.7 times the exposure in the human plasma. In contrast, the measured exposure of the male rat brain to lorcaserin at the dose at which no brain astrocytoma was observed (10 mg/kg/day) is approximately 24 times the exposure in the rat plasma.

“We estimate that humans concentrate lorcaserin in the brain to a much lower extent than do rats,” said William R Shanahan, MD, Arena's senior vice president and chief medical officer. “We believe these results may be helpful in assessing the human relevance of the observation of brain astrocytoma in the rat carcinogenicity study.”

This study is one of the activities intended to address the observation of brain astrocytoma in male rats as part of the overall plan to submit a response to the lorcaserin Complete Response Letter (CRL). Activities intended to address the observation of mammary adenocarcinoma in female rats and other issues identified by the US Food and Drug Administration (FDA) are ongoing.

Brain astrocytoma was observed in male rats given certain doses of lorcaserin during a two-year carcinogenicity study. One approach to estimating a safety margin for this finding would be to use plasma concentrations in humans at the clinically tested dose of lorcaserin and in rats at the dose of lorcaserin at which no brain astrocytoma was observed; the human plasma exposure to lorcaserin 10 mg BID is approximately five times lower than the male rat plasma exposure to lorcaserin 10 mg/kg/day.

Because lorcaserin might enter the brain differently in rats and humans, relative brain exposure may more accurately estimate the safety margin than relative plasma exposure. The apparent consistent relationship of the lorcaserin brain to CSF exposure ratios in three animal species (mice, rats and monkeys) measured in five preclinical studies conducted by Arena provides a method to estimate human brain exposure by using CSF measurements from humans and assuming a similar brain to CSF ratio found in animals.

In this clinical study, lorcaserin CSF and plasma concentrations were measured in nine healthy obese volunteers after oral administration of lorcaserin 10 mg BID for seven days. On Day 7, lumbar CSF and plasma were serially collected simultaneously over a 12-hour period. Arena calculated the estimated ratio of lorcaserin exposure in the brain relative to plasma in humans using the mean brain to CSF exposure ratio from the preclinical studies of 101, with a range of 75-117, and the measured human CSF and plasma exposures (mean AUCss (standard deviation)) of 9.3 (+/-3.9) hr-ng/mL and 540 (+/-157) hr-ng/mL, respectively, from this study.

It is important to note that Arena's estimates are based on certain assumptions and extrapolations. The FDA may accept Arena's assumptions and extrapolations or may use different ones in analysing the data, which could lead the FDA to estimate a different exposure margin. The FDA also may or may not view the estimates as reliable or predictive of the safety margin.

Lorcaserin is an investigational drug candidate intended for weight management, including weight loss and maintenance of weight loss, in patients who are obese (BMI >30) or patients who are overweight (BMI >27) and have at least one weight-related co-morbid condition. Lorcaserin is a new chemical entity that is believed to act as a selective serotonin 2C receptor agonist. The serotonin 2C receptor is expressed in the brain, including the hypothalamus, an area believed to be involved in the control of appetite and metabolism. Arena has patents that cover lorcaserin in the United States and other jurisdictions that in most cases are capable of continuing into 2023 without taking into account any patent term extensions or other exclusivity Arena might obtain.

Arena submitted a New Drug Application (NDA) for lorcaserin to the FDA in December 2009, and the FDA issued a CRL in October 2010. Arena's wholly owned subsidiary, Arena Pharmaceuticals GmbH, has granted Eisai Inc. exclusive rights to market and distribute lorcaserin in the United States subject to FDA approval of the NDA for lorcaserin.

Arena is a clinical-stage biopharmaceutical company focused on discovering, developing and commercializing oral drugs that target G protein-coupled receptors, an important class of validated drug targets, in four major therapeutic areas: cardiovascular, central nervous system, inflammatory and metabolic diseases.

Eisai Inc. is focussed on neurology, gastrointestinal disorders and oncology/critical care.