Advances in Small Molecule Inhibitors for Psoriasis Treatment

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Advances in Small Molecule Inhibitors for Psoriasis Treatment

Psoriasis affects over 125 million people worldwide—about 2-3% of the global population—making it one of the most prevalent chronic inflammatory skin conditions. While biologic drugs (targeted therapies that block cytokines like TNF-α or IL-23) have transformed care for moderate-to-severe cases, their high cost, injectable delivery, and limited access leave many patients underserved. Enter small molecule inhibitors (SMIs): oral or topical drugs that are cheaper to produce, easier to administer, and increasingly proving effective in clinical trials. Researchers from the Shanghai Skin Disease Hospital and Tongji University School of Medicine recently reviewed the latest progress in SMIs for psoriasis, highlighting their potential to bridge the gap between traditional systemic therapies (like methotrexate) and biologics.

PDE4 Inhibitors: A First Line of Small Molecule Treatment

Phosphodiesterase 4 (PDE4) inhibitors work by increasing levels of cyclic adenosine monophosphate (cAMP), a molecule that reduces inflammation. They are among the most studied SMIs for psoriasis.

Apremilast: The First FDA-Approved SMI for Psoriasis

Apremilast, approved by the U.S. FDA in 2014 for moderate-to-severe plaque psoriasis and active psoriatic arthritis (PsA), is an oral PDE4 inhibitor with a strong evidence base:

  • Efficacy: In phase IIb-III trials, 29-41% of patients taking 30 mg twice daily (b.i.d.) achieved a PASI 75 (75% reduction in psoriasis area and severity index) by week 16. The LIBERATE trial found its effectiveness comparable to etanercept, a popular biologic.
  • Real-World Use: A French study of 14,147 patients found 69% stopped apremilast within a year—higher than methotrexate’s 58% discontinuation rate—though reasons (like side effects or cost) were not specified.
  • Pediatric and PsA Benefits: A phase II trial showed apremilast is safe and effective for children with moderate-to-severe psoriasis. For PsA, 5-year data from the PALACE trials found 67% of patients achieved a 20% improvement in arthritis symptoms (ACR 20), with 27% reaching ACR 70.
  • Metabolic Perks: Case reports suggest apremilast may improve blood sugar and lipid levels, though larger studies are needed to confirm this.
  • Side Effects: Common issues include diarrhea, nausea, and upper respiratory infections. About 20% of patients lose more than 5% of their body weight with long-term use.

Hemay005: A New PDE4 Inhibitor in Development

Hemay005, a novel PDE4 inhibitor with stronger potency and fewer side effects, is in phase II trials in China for psoriasis. Early data suggest it could offer a better safety profile than apremilast.

Crisaborole: Topical Relief for Mild-to-Moderate Psoriasis

Crisaborole, already approved for atopic dermatitis, is being tested in phase II trials for psoriasis. As a topical PDE4 inhibitor, it targets inflammation directly on the skin—ideal for patients with milder disease who want to avoid oral drugs.

JAK Inhibitors: Targeting Intracellular Signaling

Janus kinase (JAK) inhibitors block the JAK-STAT pathway, a key driver of inflammation in psoriasis. Unlike biologics, which target cytokines outside cells, JAK inhibitors work inside cells to stop inflammatory signals from reaching the nucleus. The JAK family includes JAK1, JAK2, JAK3, and tyrosine kinase 2 (TYK2).

Tofacitinib: Approved for PsA, Promising for Psoriasis

Tofacitinib (targets JAK1/2/3) is approved for PsA but not yet for psoriasis. Still, phase II-III trials show strong efficacy:

  • Dose-Dependent Results: In OPT Pivotal trials, 59% of patients taking 10 mg b.i.d. achieved PASI 75 at week 12—higher than the 39% rate for 5 mg b.i.d.
  • Asian Data: A phase III study in Asia found 81% of patients on 10 mg b.i.d. reached PASI 75, compared to 55% on 5 mg b.i.d.
  • Pediatric Use: A small open-label trial found 70% of children on 5 mg b.i.d. achieved PASI 75 by week 36, though larger trials are needed.
  • Ointment Limitations: A topical tofacitinib cream showed no significant benefit over placebo at week 12—likely because the drug doesn’t penetrate skin deeply enough.
  • Safety Concerns: Tofacitinib increases the risk of herpes zoster, serious infections (like tuberculosis), and blood clots. A 2019 report of a fatal pulmonary embolism in a rheumatoid arthritis patient delayed its psoriasis approval.

Baricitinib: JAK1/2 Inhibitor for Psoriasis

Baricitinib (approved for rheumatoid arthritis) showed promise in a phase II trial: 54% of patients on 10 mg b.i.d. achieved PASI 75 at week 12, with 81% maintaining results for 24 weeks.

Filgotinib and Itacitinib: Selective JAK1 Inhibitors

Filgotinib is in phase III trials for PsA. Itacitinib, tested in a phase II trial, showed 82% of patients on 600 mg once daily (q.d.) achieved PASI 50 (50% improvement)—far higher than placebo’s 8%. However, itacitinib development has stalled.

Peficitinib: Selective JAK3 Inhibitor

A phase IIa trial found 59% of patients on 100 mg b.i.d. achieved PASI 75, with no serious side effects. Peficitinib’s selectivity for JAK3 may reduce off-target effects.

Selective TYK2 Inhibitors: A New Frontier

Early JAK inhibitors (like tofacitinib) are “pan-JAK” inhibitors—they block multiple JAK enzymes, increasing side effects. Selective TYK2 inhibitors target only TYK2, a key player in the IL-23/Th17 pathway (the main driver of psoriasis inflammation). This selectivity may offer better safety while maintaining efficacy.

Brepocitinib: Oral and Topical TYK2/JAK1 Inhibitor

Brepocitinib (targets TYK2/JAK1) modulates the IL-23/Th17 pathway as early as week 2. A phase I trial found 96% of patients on 100 mg q.d. achieved a mean PASI reduction of 96% at week 4. It’s being tested in both oral and topical forms.

BMS-986165: Potent Selective TYK2 Inhibitor

BMS-986165, which binds to TYK2’s pseudokinase domain, showed impressive results in a phase II trial:

  • PASI 75 Rates: 75% of patients on 12 mg q.d. achieved PASI 75—far higher than placebo’s 7%.
  • Safety: Common side effects included headache and diarrhea, with no serious lab abnormalities.

BMS-986165’s selectivity makes it a top candidate for future approval.

Other Small Molecules in Development

Beyond PDE4 and JAK inhibitors, researchers are testing:

  • Protein kinase C (PKC) inhibitors: Block enzymes that drive skin cell overgrowth.
  • Phosphoinositide 3-kinase δ (PI3Kδ) inhibitors: Target immune cell signaling.
  • Retinoic acid receptor-related orphan nuclear receptor γt (RORγt) inhibitors: Stop Th17 cell activation.
  • Aryl hydrocarbon receptor (AhR) modulators: Regulate immune and skin cell function.

The Future of SMIs in Psoriasis

SMIs offer a middle ground between traditional systemic drugs (like methotrexate, which has broad side effects) and biologics (which are expensive and injectable). They are oral or topical, cheaper to produce, and increasingly targeted to specific pathways—reducing off-target effects.

But challenges remain. Long-term safety data are lacking, and real-world effectiveness (beyond clinical trials) needs to be confirmed. For now, SMIs are a promising addition to the psoriasis toolbox—especially for patients who can’t access or tolerate biologics.

As the Shanghai team concludes: “Small molecule inhibitors have the potential to mediate the functions between traditional oral systemic drugs and biologics. However, further data are required to ascertain their role as effective, safe, and inexpensive alternatives.”

References

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