Abstract
Alkaline phosphatase (AP) is a gate-keeper of innate immune system responses by detoxifying inflammation triggering moieties released from endogenous and external sources. We examined whether AP's broad mechanism of action constitutes a safe therapeutic, either as single agent or combined with methotrexate (MTX), for chronic inflammatory disorders, for example, rheumatoid arthritis (RA). A rat model for RA was used with repeated intra-articular methylated bovine serum albumin (mBSA) injections in 1 knee ("arthritic" knee), with the contralateral knee serving as internal control. AP (200 µg, subcut) was administered before mBSA injections (prophylactic setting) or after arthritis induction (therapeutic setting) or combined with MTX (0.3 mg/kg or 1 mg/kg; intraperitoneally). As end point of treatment outcome, macrophage infiltration in knees, liver, and spleen was assessed by immunohistochemistry (ED1 and ED2 expression), immunofluoresence (macrophage marker folate receptor-β [FRβ]), and [18F]fluoro-polyethylene glycol-folate positron emission tomography (PET) (macrophage imaging) and ex vivo tissue distribution. Single-agent AP treatment and combinations with MTX were well tolerated. Both prophylactic and therapeutic AP markedly reduced synovial macrophage infiltration in arthritic knees (ED1: 3.5- to 4-fold; ED2: 3.5- to 6-fold), comparable with MTX treatment. AP-MTX combinations slightly improved on single agent effects. PET monitoring and ex vivo tissue distribution studies corroborated the impact of AP, MTX, and AP-MTX on reducing synovial macrophage infiltration. Beyond localized articular effects, AP also revealed systemic anti-inflammatory effects by a 2-fold reduction of ED1, ED2, and FRβ+ macrophages in liver and spleen of arthritic rats. Collectively, single-agent AP and AP combined with MTX elicited local and systemic anti-arthritic activity in arthritic rats.
Highlights
Arthritis induction in rats was associated with measurable macroscopic thickening of the arthritic knee compared with the contralateral control knee
The main finding of this study is that interventions with Alkaline phosphatase (AP) elicit prophylactic anti-arthritic activity in rats by suppressing arthritis induction after i.a. antigen injection
Tissue distribution was performed at 60 min after tracer injection for arthritic rats that had received 4 times treatment with phosphate buffered saline (PBS), AP, AP + MTX (0.3 mg/kg), and AP + MTX (1.0 mg/kg)
Summary
Rheumatoid arthritis (RA) is an auto-immune disease with hallmarks of synovial and systemic inflammation which, when left untreated, leads to progressive bone and joint destruction.[1,2,3] Current treatment options for RA include chemical disease modifying anti-rheumatic drugs (cDMARDs) with methotrexate (MTX) and glucocorticoids as predominant initial treatments.[4,5] MTX has an established place in RA treatment based on its low cost, safety profile, efficacy, and long-standing clinical experience.[6,7] Still, a considerable fraction (30%À40%) of RA patients is faced with MTX intolerance or inefficacy early on in the treatment or loss of efficacy during chronic treatment.[8,9,10,11] At this stage, treatment with biological DMARDs (bDMARDs) is indicated,[1] but these come with higher costs and are prone to development of resistance.[10,12] identification of novel modalities that could reinforce longer-lasting efficacy of MTX, or have alternative mechanisms of action, deserve continued interest. Inflammation triggering moieties (ITMs) such as adenine nucleotides (AMP, ADP, ATP) when released extracellularly, provoke a pro-inflammatory response.
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