Advances in Small Molecule Inhibitors for Treatment of Psoriasis

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Advances in Small Molecule Inhibitors for Treatment of Psoriasis

Psoriasis is a chronic, multisystemic inflammatory disease characterized by dysregulated immune responses involving cytokines such as interleukin (IL)-23, T-helper (Th) 17 pathway mediators, and tumor necrosis factor-α (TNF-α). While biologics targeting these cytokines have transformed psoriasis management, limitations such as high costs, invasive administration, and immunogenicity persist. Small molecule inhibitors (SMIs) offer a promising alternative due to their oral or topical administration, cost-effectiveness, and simpler synthesis processes. This review focuses on SMIs in clinical development or approved for psoriasis treatment, emphasizing their mechanisms, efficacy, safety profiles, and clinical trial outcomes.

Phosphodiesterase 4 Inhibitors (PDE4Is)

PDE4Is modulate intracellular cyclic adenosine monophosphate (cAMP) levels, reducing pro-inflammatory cytokine production. Apremilast, an oral PDE4 inhibitor approved by the U.S. FDA in 2014, is indicated for moderate-to-severe plaque psoriasis and psoriatic arthritis (PsA). Clinical trials demonstrated its efficacy:

  • In phase IIb–IIIb studies, 29%–41% of patients receiving apremilast 30 mg twice daily (b.i.d.) achieved ≥75% improvement in Psoriasis Area and Severity Index (PASI 75) by week 16.
  • The LIBERATE trial showed comparable efficacy between apremilast and etanercept, a TNF-α inhibitor.
  • Long-term PALACE 1–3 trials for PsA reported sustained responses, with 67.2%, 44.4%, and 27.4% of patients achieving American College of Rheumatology (ACR) 20, 50, and 70 scores, respectively, over five years.

Despite its benefits, apremilast has a high discontinuation rate (69% at one year in a French study) due to adverse events (AEs) like diarrhea, nausea, and weight loss (>5% in 20.2% of long-term users). Case studies also suggest apremilast may improve metabolic parameters, such as lipid profiles and glucose metabolism, though further validation is needed.

Hemay005, a novel PDE4I with enhanced potency and fewer AEs, is undergoing phase II trials in China. Crisaborole, a topical PDE4I approved for atopic dermatitis, showed efficacy in mild-to-moderate psoriasis and is in phase II trials for psoriasis.

Janus Kinase (JAK) Inhibitors

JAK inhibitors block signaling pathways of cytokines like IL-23, IL-17, and interferon-γ by targeting JAK1, JAK2, JAK3, or tyrosine kinase 2 (TYK2).

Pan-JAK Inhibitors

Tofacitinib, a JAK1/2/3 inhibitor approved for PsA but not psoriasis, demonstrated dose-dependent efficacy:

  • Phase II–III trials reported PASI 75 rates of 25.0%, 40.8%, and 66.7% with 2, 5, and 15 mg b.i.d., respectively, at week 12.
  • In Asian populations, PASI 75 rates were 54.6% (5 mg b.i.d.) and 81.1% (10 mg b.i.d.).
  • Pediatric studies showed 55.32% and 70.21% PASI 75 rates at weeks 12 and 36, respectively.

However, safety concerns emerged, including herpes zoster reactivation, infections, and a fatal pulmonary embolism in a rheumatoid arthritis patient. Consequently, tofacitinib’s use in psoriasis remains restricted.

Baricitinib, a JAK1/2 inhibitor approved for rheumatoid arthritis, achieved PASI 75 rates of 42.9% (8 mg) and 54.1% (10 mg) in a phase II study, with 81% of responders maintaining efficacy for 24 weeks.

Selective JAK Inhibitors

Filgotinib (JAK1-selective) and peficitinib (JAK3-selective) showed mixed results:

  • Itacitinib (JAK1 inhibitor) achieved PASI 50 rates of 81.8% (600 mg) and 66.7% (200 mg) versus 8.3% for placebo.
  • Peficitinib (100 mg b.i.d.) resulted in a 58.8% PASI 75 rate compared to 3.4% for placebo.

Brepocitinib, a dual TYK2/JAK1 inhibitor, demonstrated rapid IL-23/Th17 axis modulation and significant PASI reductions (67.92% and 96.31% with 30 mg and 100 mg daily, respectively) in phase I trials. AEs included upper respiratory tract and herpes zoster infections.

TYK2 Inhibitors

Selective TYK2 inhibitors aim to minimize off-target effects. BMS-986165, a pseudokinase domain-targeting TYK2 inhibitor, showed remarkable efficacy in a phase II trial:

  • PASI 75 rates were 75% (12 mg daily), 67%–69% (3–6 mg b.i.d.), and 39% (3 mg daily) versus 7% for placebo.

Common AEs included nasopharyngitis, headache, and diarrhea, with no significant lab abnormalities. PF-06826647, another TYK2 inhibitor, is under investigation.

Other SMIs in Development

Emerging SMIs target diverse pathways:

  • Protein kinase C (PKC) inhibitors: Modulate keratinocyte differentiation.
  • Phosphoinositide 3-kinase δ (PI3Kδ) inhibitors: Suppress immune cell activation.
  • Retinoic acid receptor-related orphan nuclear receptor gamma t (RORγt) inhibitors: Block Th17 differentiation.
  • Aryl hydrocarbon receptor (AHR) modulators: Regulate immune and keratinocyte functions.

These agents are in early-phase trials, with efficacy and safety data pending.

Clinical Implications and Challenges

SMIs bridge the gap between conventional systemic therapies (e.g., methotrexate) and biologics, offering oral/topical administration and lower costs. However, challenges remain:

  1. Safety: JAK inhibitors carry risks of infections and thromboembolic events, necessitating rigorous monitoring.
  2. Efficacy Durability: Long-term data beyond 5 years are limited for most SMIs.
  3. Patient Stratification: Identifying subgroups most likely to benefit from specific SMIs requires biomarker research.
  4. Combination Therapies: Potential synergies with biologics or topical agents warrant exploration.

Conclusion

SMIs represent a transformative advancement in psoriasis treatment, combining efficacy, convenience, and cost advantages. While PDE4Is and JAK inhibitors have established roles, next-generation agents like TYK2 inhibitors may offer enhanced specificity. Ongoing research must address safety concerns, optimize dosing, and validate long-term benefits to solidify SMIs as first-line alternatives to biologics.

doi.org/10.1097/CM9.0000000000001351

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