SAN DIEGO ( TheStreet) -- A slow launch for the weight-loss pill Belviq was somewhat expected, so before getting into details of sales reported Thursday, let's first talk about the "new" new Arena Pharmaceuticals ( ARNA) bull thesis: Bel-Phen. Arena and its partner Eisai announced plans to conduct a 12-week pilot study combining Belviq with phentermine, a currently approved weight-loss medicine. Details about the design of the study were not disclosed, although patient enrollment is expected to begin late this year or early in 2014, Arena said. Given 12 weeks of treatment, a rough guess would be top-line results announced in the second half of 2014. This catalyst now becomes the bright and shiny object of hope for Arena supporters. They believe Bel-Phen will be the next Fen-Phen weight-loss blockbuster therapy -- minus the pesky problem of melting heart valves, of course. Belviq (lorcaserin) is chemically similar to fenfluramine, except the former is not supposed to cause the serious heart defects which caused the latter to be yanked off the market. Whether or not Bel-Phen will be found to be heart safe remains to be seen. Arena shares are trading down just 1% to $6.88 Thursday morning, which I attribute to the announcement of the first Bel-Phen study and little surprise about the slow Belviq launch given close tracking of weekly prescription data. Arena reported $1.3 million in Belviq net product sales in the second quarter, which is 31.5 percent of the $4.1 million in net product sales recorded by Eisai. Belviq hit pharmacy shelves in the beginning of June, so we're only talking about a month of sales in the second quarter. Still, $4.1 million in net product sales is light, especially considering that number includes inventory stocking. Belviq's reported net sales underwhelmed, in part, because Eisai is giving away tons of free samples. It's estimated that between 60-70 percent of all Belviq prescriptions filled are gratis. Separately, Eisai reported $10 million in gross sales of Belviq, which means the gross-to-net discount was a whopping 60 percent.
Arena says the 60 percent gross-to-net discount will fall in subsequent quarters as more patients switch from free Belviq and start to pay for the drug. Hopefully. How much of the $4 million in net Belviq sales came from patients actually using the drug versus bottles sitting in a distributor's warehouse? We can do the rough math. Eisai said it shipped 50,000 bottles of Belviq to distributors. At $200 per bottle, that's $10 million in gross sales. Arena said 12,500 prescriptions for Belviq were filled between June 11 and July 19, according to IMS Health. At $200 per script, those filled scripts generated $2.5 million in patient-derived Belviq revenue. But remember, approximately 60-70 percent of Belviq scripts are free or discounted, so the real Belviq end-user revenue is closer to $1 million in the second quarter -- or a quarter of the reported $4.1 million in net product sales. Rough math, acknowledged. Arena would not separate out actual Belviq end-user sales from sales to distributors and wholesalers. We do know that IMS Health is doing a decent job of tracking Belviq scripts accurately. IMS weekly reports between June 11 and July 19 had 10,538 Belviq prescriptions filled, equating to 84 percent of the 12,500 scripts reported by Arena on Thursday. -- Reported by Adam Feuerstein in Boston. Follow @AdamFeuerstein