Mechanistic distinction, clinical signal comparison, and the off-treatment durability gap. Indication scope: 2L+ adult chronic ITP. May 2026 refresh incorporates Wayrilz (rilzabrutinib) FDA approval (2025-08-29) as a third mechanistic class.
Section 1 - Executive summary
RCTRandomised controlled trial. Highest evidence.IndirectMechanistic inference or cross-trial comparison. No head-to-head RCT.RWEReal-world evidence. Hypothesis-generating.EditorialAnalytical interpretation. Not a published conclusion.Indirect.
| Domain | TPO-RAs | FcRn inhibitors | Rilzabrutinib (BTKi) | Verdict |
|---|---|---|---|---|
| Mechanism layer | Downstream - stimulates platelet production | Upstream - reduces pathogenic IgG catabolism | Dual - B-cell BTK + macrophage FcγR signalling | Three distinct targets |
| On-treatment response | 38-79% RCT | 5-22% RCT | 64% initial 12w response RCT | TPO-RA + BTKi lead |
| Durable response (per-trial primary endpoint) | 52% continuous response ≥25w on-treatment (EXTEND OLE) RCT | 22% sustained platelet response ≥4 of last 6w (ADVANCE IV) RCT | 23% ≥50×10⁹/L on ≥8 of last 12w, vs 0% pbo (LUNA 3) RCT | Three distinct endpoints; not directly comparable |
| Time to response | 1-2 weeks RCT | 7-8 days RCT | 15 days (responders) RCT | FcRn fastest |
| Off-treatment remission | 30-33% RCT+RWE | Not published | Not published; OLE ongoing | Data gap - FcRn, BTKi |
| ADA formation | Minimal (oral small molecule) Indirect | Rozanolixizumab 71-75% RCT | N/A (oral small molecule) Indirect | TPO-RA, BTKi advantage |
| Disease modification | Not proven Editorial | Not proven Editorial | Not proven Editorial | No class proven |
Table 1.1 - Summary comparison. All cross-class comparisons are Indirect (no head-to-head RCT exists). The "Durable response" row spans three incommensurable primary endpoints (EXTEND continuous ≥25w on-treatment; ADVANCE IV ≥4 of last 6w; LUNA 3 ≥8 of last 12w). Numbers are not directly comparable. Rilzabrutinib column added May 2026 following Wayrilz FDA approval (NDA 219685, 2025-08-29).
Section 3 - Clinical signal comparison (excerpt)
| Agent | Class | Key trial | Primary endpoint | Response | Evidence |
|---|---|---|---|---|---|
| TPO receptor agonists | |||||
| Eltrombopag | TPO-RA | RAISE (Ph 3) | ≥50×10⁹/L at least once (6mo) | 79% (vs 28% pbo, p<0.0001) | RCT |
| Eltrombopag | TPO-RA | EXTEND (Ph 3 OLE) | Continuous response ≥25w on-treatment | 52% | RCT |
| Eltrombopag | TPO-RA | TAPER (Ph 2) | Sustained off-treatment response ≥26w | 30.5% | RCT |
| Romiplostim | TPO-RA | Kuter 2008 (Ph 3) | Durable response ≥50×10⁹/L for ≥6/8w | 38-61%* (vs 0-5% pbo) | RCT |
| Avatrombopag | TPO-RA | Ph 3 (NCT01438840) | Cumulative weeks ≥50×10⁹/L without rescue | 65.6% at Day 8 (vs 0% pbo) | RCT |
| FcRn inhibitors | |||||
| Efgartigimod | FcRn | ADVANCE IV (Ph 3) | Sustained ≥50×10⁹/L for ≥4 of last 6 weeks | 22% (vs 5% pbo, p=0.032) | RCT |
| Rozanolixizumab | FcRn | TP0003 (Ph 3, terminated) | Durable ≥50×10⁹/L for ≥8/12w | 19% (4/21) | RCT Underpowered |
| Rozanolixizumab | FcRn | TP0006 (Ph 3, terminated) | Durable ≥50×10⁹/L for ≥8/12w | 5% (1/20) | RCT Underpowered |
| Nipocalimab | FcRn | - | - | No ITP trial registered on ClinicalTrials.gov (May 2026) | No data |
| BTK inhibitors (third class, added May 2026) | |||||
| Rilzabrutinib (Wayrilz) | BTKi | LUNA 3 (Ph 3, NCT04562766) | Durable platelet response ≥50×10⁹/L ≥8/12w | 23% (rilzabrutinib) vs 0% pbo | RCT |
| Rilzabrutinib (Wayrilz) | BTKi | LUNA 3 - 12w initial | Platelet response (any) | 64% rilzabrutinib; rescue 33% vs 58% pbo | RCT |
*38% splenectomised, 61% non-splenectomised. Rozanolixizumab trials terminated early at 30-40% of planned enrolment - formally underpowered. Rilzabrutinib FDA-approved as Wayrilz (NDA 219685) on 2025-08-29 for persistent or chronic ITP with insufficient response to prior therapy; LUNA 3 is the pivotal trial. All cross-class comparisons are Indirect.
Durability is the lens through which both classes must ultimately be judged. The data are strikingly asymmetric - not because one class is clearly superior, but because the comparison cannot yet be made.
| Metric | TPO-RAs | FcRn inhibitors |
|---|---|---|
| On-treatment "continuous response" | 52% (EXTEND, ≥25w) - on-treatment only RCT | 22% (ADVANCE IV, ≥4/6w) - on-treatment only RCT |
| Off-treatment remission (published) | 30.5% (TAPER, ≥26w off drug) RCT | Not published in any Phase 3 ITP trial No data |
| Relapse kinetics post-cessation | Gradual; 70% relapse within 2-4 weeks RWE | Likely rapid (IgG rebounds 3-5w post-dose); not formally measured Indirect |
| Re-treatment response maintained? | Yes - ~70-80% re-respond (REPEAT trial) RCT | Unknown - not studied No data |
| Data maturity | 15+ years Phase 3 + RWE RCT+RWE | <3 years Phase 3 data; OLE ongoing RCT |
Table 3.4 - Durability comparison. Off-treatment remission is the only unambiguous measure. For FcRn inhibitors, this metric is currently unpublished.
Three scenarios, three exact statements. Each is defensible at the evidence-tier shown and stays on label.
Scenario 1. Hematologist asks: "Which class gives more durable response?"
Statement. "On-treatment, TPO-RAs show 38-79% response (RAISE, Kuter 2008) versus 5-22% for FcRn inhibitors (ADVANCE IV, TP0003/6). Off-treatment durability has been formally measured for TPO-RAs at 30.5% sustained remission (TAPER, RCT). For FcRn inhibitors, off-treatment durability is not yet a published Phase 3 endpoint - ADVANCE NEXT is designed to address this gap." RCT
Scenario 2. Competitor MSL cites TAPER off-treatment remission as a class superiority claim.
Statement. "TAPER establishes off-treatment remission for eltrombopag specifically. It does not generalise across the TPO-RA class - romiplostim has not demonstrated equivalent off-treatment data. FcRn inhibitors target a different mechanistic layer (pathogenic IgG catabolism), and their durability question is open, not unfavourable." RCT Indirect
Scenario 3. Hematologist asks: "Why use FcRn before durability is proven?"
Statement. "FcRn inhibition addresses the upstream destruction signal - lowering pathogenic IgG, which is the proximate driver of platelet loss in ITP. On-treatment, the response is fast (7-8 days) and biologically targeted. The off-treatment durability question is the next data readout, not a contradiction of the mechanism." Editorial
Scenario 4 (May 2026 addition). Hematologist asks: "Where does Wayrilz (rilzabrutinib) fit relative to TPO-RA and FcRn?"
Statement. "Wayrilz was FDA-approved on 2025-08-29 (NDA 219685) for persistent or chronic ITP after insufficient response to prior therapy. The pivotal trial is LUNA 3 (NCT04562766): 23% durable response ≥50×10⁹/L for ≥8 of 12 weeks vs 0% placebo. It acts on two layers simultaneously - BTK in B cells and macrophage FcγR signalling - distinct from TPO-RA (platelet production) and FcRn (IgG catabolism). Initial 12-week response is 64%; durable response is 23%. Off-treatment durability has not yet been published. Cross-class durability comparisons remain Indirect and currently lack a head-to-head RCT." RCT Indirect
Off-label boundary. Claiming FcRn-class or BTKi-class equivalence to TPO-RA durability is not yet supportable. ADVANCE next (NCT06544499) and the rilzabrutinib OLE are the next data gates; until then, the open-question framing above is the scientifically defensible position.
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