EN3835

Endo is testing EN3835, an injectable collagenase enzyme, in Phase 3 for plantar fibromatosis (also called Ledderhose disease), a benign condition in which painful collagen nodules grow inside the plantar fascia, the dense connective tissue band on the bottom of the foot [1]. The same molecule is already FDA-approved as Xiaflex for hand contractures (Dupuytren's) and penile curvature (Peyronie's disease), so this is a label expansion of a known drug, not a novel compound [2]. Phase 3 NCT06151197 enrolled 418 patients, is active but no longer recruiting, and lists an estimated primary completion date of March 31, 2026, with a 12-week pain endpoint readout expected then [1]. There is no FDA-approved drug therapy for Ledderhose disease today - patients get steroid injections, radiotherapy, or surgery - so a positive trial would create a drug category. The central question is whether the same enzyme that softens palmar (palm) cords reduces foot pain enough to clearly beat a sham injection, especially after the Phase 2 trial missed its primary endpoint in the overall population [9].

EN3835 is the same active ingredient as Xiaflex/Xiapex, Endo's collagenase clostridium histolyticum, approved for Dupuytren's contracture in 2010 and Peyronie's disease in 2013 [2]. So this is a label expansion play, not a novel chemical entity. No public FDA designations apply to the Ledderhose indication: no breakthrough therapy, fast track, or orphan tag has been disclosed for EN3835 in plantar fibromatosis. Phase 3 NCT06151197 sits at active not recruiting with n=418 and primary completion estimated March 31, 2026 [1]; the long-term safety and retreatment study NCT05254457 closed at n=145 [3]; Phase 2 NCT05152173 wrapped at n=176 [4]. Endo also ran a separate Phase 2 in plantar fasciitis (NCT06169319, n=231, completed), suggesting the company is testing the foot-pain hypothesis broadly across fascia indications [8]. Endo International plc emerged from Chapter 11 on April 23, 2024, with substantially all assets sold to a new entity, Endo, Inc., owned mostly by former first-lien debt holders [5]. EN3835 is one of the higher-visibility pipeline assets the reorganized company emerged with, which sharpens commercial stakes around the Phase 3 readout.

Collagenase clostridium histolyticum (CCH) is a mix of two bacterial enzymes, AUX-I and AUX-II, purified from Clostridium histolyticum. They cut triple-helical collagen, the structural protein that makes scar tissue and fibrotic cords rigid. In plantar fibromatosis, abnormal fibroblasts (the cells that deposit collagen) lay down dense bundles of type I and type III collagen in the plantar fascia, forming firm nodules that press on nerves and produce sharp pain on weight-bearing. Note this is distinct from plantar fasciitis, which is inflammation of the plantar fascia where it attaches to the heel - same anatomic structure, different pathology. The drug is injected directly into the fibromatosis nodules; the enzymes hydrolyze collagen, the nodule softens, and mechanical pressure on surrounding tissue drops. Mechanism is well-validated for Dupuytren's contracture in the hand, where CCH dissolves palmar cords (the tight fibrous bands in the palm that contract the fingers) enough to permit manipulation and cord rupture [2]. Whether the same trick works on the foot is the real question. The plantar fascia is thicker than palmar fascia, weight-bearing, and Ledderhose nodules tend to be more diffuse than the discrete cords seen in Dupuytren's. After enzymatic breakdown, you may not get the clean mechanical release that drives the Xiaflex response in the hand. The biology of the target collagen is identical; the geometry and mechanics of the tissue are not.

NCT06151197 is a Phase 3 placebo-controlled study in 418 patients with plantar fibromatosis sponsored by Endo [1]. Primary endpoint is change from baseline to Week 12 in the weekly mean of average daily pain on a numeric rating scale (NRS). Status: active, not recruiting; enrollment complete; estimated primary completion March 31, 2026. The Phase 2 (NCT05152173) used up to two CCH injections per nodule separated by a minimum 28-day interval, with the Foot Function Index (FFI) pain subscale at Day 57 as primary [4][9]. Critically, Phase 2 missed that primary endpoint in the overall population (n=176): CCH did not separate from placebo on FFI pain at Day 57 [9]. The trial did meet secondary endpoints - Clinician Global Impression of Change and nodule consistency/hardness - and a post-hoc subgroup analysis found CCH significantly improved FFI pain and combined pain-plus-difficulty scores in a subset of patients, which Endo has presented at podiatry meetings [10]. This is the basis the Phase 3 was designed against, but post-hoc subgroup wins do not de-risk a confirmatory Phase 3 the way a clean primary hit would. The Phase 3 swapped FFI for the more interpretable NRS and lengthened follow-up to Week 12. The comparator is a sham saline injection. Placebo risk is real: in pain studies of injectable interventions, placebo response can be large because the procedure itself may deliver mechanical effect on its own.

Our model gives this drug a 29% chance of eventually being approved. That number starts from the historical approval rate for Phase 3 drugs in this area, which is about 69%, then adjusts based on ten specific facts about the trial and its sponsor. The biggest positive factor is having more secondary endpoints than usual; the biggest negatives are the sponsor's weak approval record, limited earlier-phase results, and a randomized trial design. The remaining factors were close to average and left the estimate roughly where it started.

Efficacy risk dominates and is materially worse than the original model captured. Phase 2 missed its primary endpoint in the overall population [9], and a confirmatory Phase 3 powered against a post-hoc subgroup effect is a known failure pattern. Injecting an enzyme that chews up collagen into a weight-bearing fibrotic nodule may shrink the nodule, but pain reduction depends on whether nodule mass was actually driving pain versus inflammation, neural compression, or biomechanical strain in the surrounding fascia. Placebo response in injection trials for foot pain commonly runs in the 30-40% range, which makes effect-size separation hard. Safety risk: known on-target effects of CCH (side effects from the drug's intended mechanism, not off-target toxicity) include injection-site bruising, swelling, and pain, plus rare tendon ruptures when the enzyme spreads beyond target tissue [2]. The plantar fascia sits adjacent to flexor tendons and attaches to the calcaneus (heel bone); if collagenase leaks into a tendon, you get partial or complete rupture, plus a foreseeable liability problem. The Qwo precedent matters: Endo voluntarily ceased Qwo production December 6, 2022 because injection-site bruising was so extensive and variable, and skin discoloration so prolonged, that the market rejected the product despite FDA approval - Endo had even run a dedicated mitigation study (APHRODITE) that failed to adequately reduce bruising [7]. The plantar nodule tissue is dense fibrous tissue versus Qwo's subcutaneous fat, so the analogy is imperfect, but the regulatory and commercial precedent on CCH tolerability outside the hand is directly relevant. (A reported Qwo FDA 'boxed warning' in early 2022 could not be verified in public sources, so only what is documented is stated here.) Commercial risk: Ledderhose prevalence estimates run 0.3-6% depending on geography and definition, the treated population is modest, and post-bankruptcy Endo faces pricing pressure from cheap steroid alternatives. IP risk: the core Xiaflex composition-of-matter patents are expected to expire around 2027 per public patent watch sources [11], meaning even if EN3835 wins approval, the pricing-power runway against potential biosimilar collagenase is short.

Worth watching, but the editor was right to push on the Phase 2 record - this is more downside-skewed than the original 51.8% POS implied. EN3835 is a label expansion of a well-characterized enzyme into an indication with no approved drug option, run by a post-bankruptcy company that needs commercial wins. The specific catalyst: Phase 3 NCT06151197 estimated primary completion March 31, 2026, with top-line pain data presumably within the following 3-6 months, and whether effect size on the 12-week NRS endpoint cleanly separates from sham [1]. A 1.5-2 point improvement on a 10-point NRS gets approved; under 1 point gets eaten by placebo. Market frame: at 0.3-6% prevalence (range is wide), even a midpoint ~1.5% of ~258M US adults implies ~3.9M with the condition, of whom likely <10% would be drug-treatment candidates with painful symptomatic nodules at a given time. Anchoring price to Xiaflex (publicly reported ~$3,000-4,000 per injection course in Dupuytren's), a plausible peak US market sits in the low hundreds of millions per year if uptake is meaningful - modest by big-pharma standards, meaningful for a reorganized Endo. Pre-2027 patent runway compresses that further [11]. Endo is private post-restructuring, so financial disclosures will be limited; earnings calls are gone, but trial milestones still get pressed [5]. The mechanism is solved; clinical execution is the variable. Either the pain endpoint hits or it doesn't, and the direction will be apparent from the first press release. A positive readout meaningfully helps Endo's reorganization narrative and creates a small but defensible drug category. A negative readout - and the Phase 2 primary miss says that's a real risk - leaves Endo with one fewer pipeline win and the procedural standard of care unchallenged.