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Regulus Therapeutics (RGLS) - Get Free Report has found it difficult to convince investors that an injectable hepatitis C drug could find a place in a market dominated by convenient and highly effective pills, but promising clinical data announced Wednesday strengthens the company's argument.

A four-week course of treatment consisting of two injections of Regulus' RG-101 combined with four weeks of currently approved oral medicines reduced the level of hepatitis C virus to undetectable levels in 97% of patients after eight weeks of follow-up, the company said.

Regulus shares were up 23% to $7.71 in trading on Wednesday.

Today, drugs sold by Gilead Sciences (GILD) - Get Free Report , AbbVie (ABBV) - Get Free Report and Merck (MRK) - Get Free Report cure nearly all hepatitis C patients with a single, daily pill taken for eight to 12 weeks. These drugs are so effective and safe there's little room for improvement.

Regulus, however, said it believes the RG-101 data announced Wednesday are strong enough to pursue the development of an even shorter, four-week hepatitis C regimen. Combining injections and pills could be more convenient -- and perhaps less expensive -- than current treatment options, the company argued.

In the phase II study, 38 patients with untreated genotype 1 or 4 hepatitis C were injected with RG-101, given four weeks of Harvoni, Olysio or Daklinza (all currently approved hepatitis C drugs) and then a second RG-101 injection. Eight weeks after the treatment ended, 37 of the 38 patients had hepatitis C viral loads below the limit of quantification.

Fourteen of the patients have been followed for 12 weeks and all still have undetectable hepatitis C viral loads. Two patients in the study reported serious adverse events, possibly related to RG-101.

Wednesday's data are encouraging, but significant questions and hurdles remain.

These results are preliminary because Regulus enrolled 79 hepatitis C patients in the study. Final results will come later this year after all the patients are followed for 12 weeks.

On a conference call Wednesday, Regulus said regulators may want patient follow-up longer than 12 weeks to prove RG-101 injections can provide an effective, safe and durable cure for hepatitis C.

The theoretical risk with very short treatment regimens is the hepatitis C virus rebounding so patients who might have been cured initially suffer relapses. The relapse risk for Regulus could be exacerbated because published data show some hepatitis C can become resistant to RG-101.

Even if Regulus can demonstrate the efficacy and safety of a short course of RG-101 plus pills is on par with longer treatment schedules of pills alone, the commercial opportunity might be limited.

Treatments like Gilead's Harvoni have become so entrenched in the hepatitis C market that convincing doctors and patients to switch to injections won't be easy. And while Gilead's Harvoni is almost 100% effective at eight and 12 weeks, the company is working on new therapies to shorten treatment to six weeks, thereby blunting Regulus' convenience argument.

Regulus may find its best development option for RG-101 is partnering with an existing hepatitis C company losing out to Gilead. Sales of Johnson & Johnson's (JNJ) - Get Free Report Olysio have plunged since Harvoni was launched. A combination of RG-101 and Olysio might benefit J&J, if the company decides to ratchet up its investment in hepatitis C.

On its call, Regulus executives expressed interest in finding help to further develop RG-101, although an exclusive partnering deal isn't necessarily the goal. A new study combining RG-101 with an experimental drug from


(GSK) - Get Free Report

will start soon.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.