Arrowhead says RNA drug doubled tirzepatide's weight loss in small early trial
Very early data suggest adding an experimental RNA drug to tirzepatide may roughly double weight loss and cut stored fat faster, but it would mean a second injection and its price is not yet known.
Arrowhead Pharmaceuticals said its experimental RNA-based drug roughly doubled the weight loss produced by tirzepatide alone in a small, early-stage trial, according to results the company released on Jan. 6, 2026 [2]. Arrowhead's shares rose 17%, to $75.20, when markets opened that day [2].
Tirzepatide is the molecule in Eli Lilly's Zepbound, approved for weight loss, and Mounjaro, approved for type 2 diabetes.
Four patients with type 2 diabetes who took Arrowhead's ARO-INHBE plus tirzepatide lost 9.4% of their body weight at week 16, while five patients who took tirzepatide alone lost 4.8%, the company said [2]. Arrowhead described that as a doubling of the weight loss achieved with tirzepatide by itself [2].
The company also said ARO-INHBE tripled the reduction in visceral fat, total fat and liver fat compared with tirzepatide, results that came from just three patients [2]. Given without tirzepatide, ARO-INHBE cut visceral fat by 9.9% and liver fat by 38%, and increased total lean tissue by 3.6%, with a placebo-adjusted mean visceral fat reduction of 15.6% at week 24 [2].
The data come from two Phase I/IIa trials testing two Arrowhead RNA interference candidates, ARO-INHBE and ARO-ALK7. The company said ARO-ALK7 reduced adipose ALK7 mRNA by 88% at week eight and produced a placebo-adjusted 14.1% reduction in visceral fat [2].
On safety, Arrowhead said treatment-emergent adverse events were mild and none led to discontinuations [2]. There was one report of a limb abscess, which was treated with drainage and considered unrelated to treatment [2]. Gastrointestinal side effects were similar in the combination and tirzepatide-alone groups, and lab tests showed no "clinically significant" elevations in liver enzymes, glycemic indices or lipid parameters, according to the company [2].
Truist Securities called the weight loss achieved by Arrowhead's drug "substantial" and "meaningful," and said the results could reignite interest in the quality of weight loss, not just the amount [2]. Other therapies aimed at body composition include Scholar Rock's apitegromab, Lilly's bimagrumab and Wave Life Sciences' WVE-007, according to Truist [2].
Barron's reported that cost and convenience could limit demand for the combination, and that it remains an open question how much patients will care about the extra weight loss [1].
Why it matters for patients
ARO-INHBE is an experimental drug. It is not approved by the FDA and is not available by prescription, so nothing about this report changes what is currently prescribed for weight loss or type 2 diabetes.
The numbers are small and early. The weight-loss comparison involved four patients on the combination and five on tirzepatide alone, and the fat measurements came from three patients, all followed for 16 weeks [2]. Whether the extra weight loss continues beyond that point, or lasts after treatment stops, has not been reported.
The combination is also injectable, meaning it would add a second shot on top of an existing injectable. Arrowhead has not disclosed what it would charge, and no price or insurance coverage information has been reported [1].
On tolerability, the company reported no discontinuations and gastrointestinal side effects similar to tirzepatide alone in this small group [2]. Those findings come from a handful of patients and would need confirmation in larger studies before they could be considered reliable.
What happens next
Arrowhead said both Phase I/IIa trials are continuing and that it expects more results later this year [2]. The company held a conference call at 11:30 a.m. ET on Jan. 6, 2026 [2].
Separately, Lilly is awaiting FDA approval of its oral weight loss drug orforglipron, which could launch as early as March if the agency gives the green light, according to Truist [2].
Sources
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