Cardiologists are less likely to advocate for insurance coverage for drugs to treat their patients compared to oncologists, dermatologists and rheumatologists, who have more experience doing so. This may slow sales growth for Entresto, Novartis’s new heart-failure drug, company executives said.

In a third-quarter earnings call on Tuesday, Novartis revealed that the drug generated $16 million in net sales in its first quarter on the market. Entresto received FDA approval in July to treat patients with the most serious form of chronic heart failure.

Entresto’s sales were modest in the quarter, David Epstein, Novartis Pharmaceuticals’ division head, told investors. He added, however, that the drugmaker has received positive qualitative feedback about the therapy from physicians and patients.

Insurance reimbursement remains one of the biggest roadblocks to adoption. About 65% of the eligible patients for Entresto are covered by Medicare Part D, which did not provide any reimbursement until Oct. 1. The drugmaker said it expects to receive more formulary coverage during the next few months.

Another challenge is educating cardiologists, who have traditionally prescribed older, generic (and cheaper) drugs to their patients. ACE inhibitors and angiotensin receptor blockers are the current standards of care in the treatment of heart failure.

Cardiologists “haven’t had to spend the time and build the office staff” necessary to advocate for insurers to cover a drug, Epstein said. “We have to create an even greater sense of urgency on the part of the physician,” he added.

Novartis executives said they do not plan to broaden marketing to primary care physicians until the cardiologists are on board. About 1,300 physicians have prescribed Entresto, Novartis said.