Pevita Pearce
PHOENIX — In the high-stakes arena of cardiovascular drug development, the pursuit of a novel therapeutic for heart failure has encountered a significant, albeit instructive, roadblock. Researchers investigating tovinontrine, a phosphodiesterase-9 (PDE9) inhibitor, recently announced that two pivotal Phase II placebo-controlled trials failed to meet their primary efficacy endpoints. However, the data revealed a startling paradox: the drug appears to work better at lower doses than at the higher levels originally hypothesized to be effective.
The findings, presented by James Udelson, MD, of Tufts Medical Center, at the Heart Failure Society of America (HFSA) annual meeting, suggest that the research community may have fundamentally misunderstood the dosing requirements for this specific class of medication.
The Promise of PDE9 Inhibition
To understand the significance of these trials, one must first understand the mechanism of PDE9. Phosphodiesterases are enzymes that break down cyclic nucleotides, such as cyclic guanosine monophosphate (cGMP). In the context of heart failure, cGMP is a vital signaling molecule that promotes vasodilation and protects the heart from the maladaptive remodeling that occurs during the progression of chronic disease.
PDE9 expression is known to be upregulated in heart failure, particularly in cases of heart failure with preserved ejection fraction (HFpEF). By inhibiting PDE9, investigators hope to boost intracellular cGMP levels, thereby improving cardiac function. The concept was recently bolstered by the CARDINAL-HF trial, which provided "proof of mechanism" in 60 HFrEF patients, demonstrating that a PDE9 inhibitor could successfully elevate cGMP without significant safety concerns, even when administered alongside standard-of-care medications like sacubitril/valsartan (Entresto).
Chronology of the CYCLE Trials
The development program for tovinontrine, designated under the "CYCLE" umbrella, was designed to evaluate the drug’s efficacy in both HFrEF (reduced ejection fraction) and HFpEF (preserved ejection fraction).
The CYCLE-1 Trial (HFrEF)
CYCLE-1 was a robust, international Phase II study involving 529 participants. The criteria for enrollment were strict: patients had an ejection fraction of 40% or lower, elevated NT-proBNP levels (at least 600 pg/mL), and stable New York Heart Association (NYHA) class II or III symptoms. The patient population was well-managed, with 88% on triple guideline-directed medical therapy (GDMT) and 60% on quadruple therapy.
The primary goal was to observe a reduction in NT-proBNP—a key biomarker of cardiac wall stress—at 12 weeks. Researchers randomized patients to receive either placebo or varying doses of tovinontrine, ranging from 2.5 mg to 50 mg twice daily (BID).
The CYCLE-2 Trial (HFpEF)
Running in parallel, CYCLE-2 mirrored the methodology of the first study but focused on the HFpEF cohort, defined as patients with an ejection fraction greater than 40% and less than 60%. This study enrolled 277 patients who were generally older and on slightly less intensive medical regimens than those in the HFrEF cohort. Like its predecessor, CYCLE-2 aimed to reduce NT-proBNP levels using a 50 mg BID dose.
Supporting Data: The Inverse Dose Response
The results from both trials were, at first glance, discouraging. However, upon closer inspection of the data, the investigators uncovered a phenomenon that flipped the script on traditional pharmacological expectations.
In CYCLE-1, the high dose of 50 mg BID resulted in a statistically nonsignificant 4.4% reduction in NT-proBNP compared to placebo. Surprisingly, the medium dose (25 mg BID) showed a more promising 10.8% reduction.
Even more revealing was the "inverse dose response" identified in the post-hoc analysis. The median change in NT-proBNP from baseline was most favorable at the lowest dose tested (2.5 mg BID), where patients saw a 16% reduction. As the dosage increased, the efficacy decreased: a 6% reduction at 25 mg BID, and only a 1% reduction at the 50 mg BID mark.
In CYCLE-2, the high dose of 50 mg BID failed to move the needle in the desired direction. In fact, NT-proBNP levels in the tovinontrine group actually increased by 9.4% relative to the placebo group, which saw a 10.1% reduction.
"Did we have the wrong dose? Yes, that’s pretty clear," Dr. Udelson stated during his presentation. The data suggests that for tovinontrine, "more" is not necessarily "better."
The "Ceiling Effect" Hypothesis
The most compelling implication of these findings is the existence of a biological "ceiling." Dr. Udelson proposed that there is a threshold for how much myocardial cGMP can be elevated before the body initiates compensatory mechanisms that counteract the drug’s beneficial effects.
Post-hoc analysis supported this theory. Patients who experienced only small, modest increases in plasma cGMP—achieved through lower doses of tovinontrine—exhibited more consistent reductions in NT-proBNP. Conversely, the high doses likely triggered a feedback loop that negated the therapeutic potential of the drug.
This suggests that the "sweet spot" for PDE9 inhibition is far narrower than previously assumed, requiring a delicate balance between pharmacological activity and the heart’s homeostatic response.
Safety and Tolerability
Despite the lack of efficacy at the 50 mg dose, the safety profile of tovinontrine remained largely intact. Throughout both the CYCLE-1 and CYCLE-2 trials, adverse events were generally mild. While there was a slight excess of treatment-emergent adverse events in the tovinontrine groups, these did not manifest as significant hemodynamic instability.
Blood pressure changes were minimal at the 12-week mark. Furthermore, observed renal changes were described as transient and reversible upon the discontinuation of the drug, requiring no additional medical intervention. These safety outcomes are crucial, as they provide a solid foundation for the drug’s continued development, provided the dosing strategy can be corrected.
Future Implications: The Path to CYCLE-3
The failure of the 50 mg dose has not spelled the end for tovinontrine. Instead, the research team is pivoting. A new study, CYCLE-3, is slated to begin in 2027. This Phase IIb trial will explicitly test the hypothesis that lower doses of tovinontrine—which induce less intense PDE9 inhibition and more modest cGMP elevation—will yield superior clinical outcomes.
If CYCLE-3 successfully demonstrates that lower doses lead to a meaningful reduction in NT-proBNP in HFrEF patients, the investigators intend to revisit the HFpEF population with a similarly refined dosing regimen.
The implications for the field are profound. PDE9 inhibition remains a highly attractive target, particularly because no PDE9 inhibitor currently exists in clinical practice. The field has learned a hard, yet valuable, lesson in precision pharmacology: the assumption that higher doses equate to higher efficacy is a trap in complex biological systems.
As investigators prepare for the next phase of research, the cardiovascular community remains cautiously optimistic. The success of tovinontrine may ultimately depend not on developing a more potent drug, but on finding the wisdom to use less. The future of heart failure treatment may hinge on this shift in perspective, marking a transition toward more nuanced, physiologically optimized dosing strategies that respect the inherent limitations of the human heart’s signaling pathways.
For now, the medical community will await the 2027 launch of CYCLE-3, watching to see if the "less is more" approach can finally bring this innovative therapy to the patients who need it most.
Nila Kartika Wati
Neng Nana
