A new class of recombinant human albumin with multiple surface thiols exhibits stable conjugation and enhanced FcRn binding and blood circulation

Human serum albumin is an endogenous ligand transport protein whose long circulatory half-life is facilitated by engagement with the human cellular recycling neonatal Fc receptor (hFcRn). The single free thiol located at Cys-34 in domain I of albumin has been exploited for monoconjugation of drugs. In this work, we increased the drug-to-albumin ratio potential by engineering recombinant human albumin (rHSA) variants with varying hFcRn affinity to contain three free, conjugation-competent cysteines. Structural analysis was used to identify positions for cysteine introduction to maximize rHSA stability and formation of the conjugated product without affecting hFcRn binding. The thiol rHSA variants exhibited up to 95% monomeric stability over 24 months and retained hFcRn engagement compared with a WT unconjugated control demonstrated by Biolayer Interferometry. The additional cysteines were further introduced into a panel of rHSA variants engineered with different affinities for hFcRn. After conjugation with three Alexa Fluor 680 (AF680) fluorophores, hFcRn binding was similar to that of the original triple-thiol nonconjugated rHSA variants (0.88 and 0.25 μm for WT albumin with or without 3xAF680 respectively, and 0.04 and 0.02 μm for a high hFcRn-binding variant with or without 3xAF680, respectively). We also observed a 1.3-fold increase in the blood circulatory half-life of a high hFcRn-binding triple-thiol variant conjugated with AF680 (t _½ = 22.4 h) compared with its WT counterpart (t _½ = 17.3 h) in mice. Potential high drug-to-albumin ratios combined with high hFcRn engagement are attractive features of this new class of albumins that offer a paradigm shift for albumin-based drug delivery.

Cellular recycling-driven in vivo half-life extension using recombinant albumin fusions tuned for neonatal Fc receptor (FcRn) engagement

Recombinant albumin-drug genetic fusions are an effective technology to prolong the serum half-life of therapeutics that has resulted in marketed products. Indirect evidence suggests albumin fusions' long circulation is controlled by engagement with the cellular recycling neonatal Fc receptor (FcRn) in addition to reduced kidney filtration. In this work, we have used a panel of recombinant fusions, engineered with different human FcRn (hFcRn) affinity, including a novel high binding albumin variant (HBII), to directly define and importantly, control the intracellular mechanism as a half-life extension tuning method. mNeonGreen or mCherry fusion to the N-terminal of the recombinant human albumin (rHA) variants null-binder (rHA NB), wild-type (rHA WT), high-binder I (rHA HBI), and high-binder II (rHA HBII) did not generally interfere with hFcRn interaction determined by Biolayer Interferometry. Co-localisation of the albumins with endosomal, but not lysosomal, markers was shown by confocal microscopy for high, but not low, hFcRn binders in a human microvascular endothelial hFcRn overexpressing cell line (HMEC-1 FcRn) suggestive of endosomal compartmentalisation. Furthermore, a cellular recycling assay revealed increased recycling of albumin fusions for the high binding variants (mNeonGreen WT; ~1, mNeonGreen HBI; 5.26-fold higher, and mNeonGreen HBII; 5.77-fold higher) in the hFcRn overexpressing cell line. In vivo experiments demonstrated a direct in vitro recycling/in vivo half-life correlation with a longer circulation for the mCherry fusions engineered with high hFcRn affinity that was highest with the HBII variant of 30.1 h compared to 18.2 h for the mCherry WT. This work gives the first direct evidence for an FcRn-driven endosomal cellular recycling pathway for recombinant albumin fusions that correlates with half-life extension controlled by the affinity to hFcRn; promoting a versatile method to tune the pharmacokinetics of albumin fusion-based therapeutics not met by current technologies.

CXCL13 predicts disease activity in early rheumatoid arthritis and could be an indicator of the therapeutic 'window of opportunity'

Introduction

A key phenomenon in rheumatoid arthritis is the formation of lymphoid follicles in the inflamed synovial membrane. C-X-C motif chemokine 13 (CXCL13) is central in this process as it attracts C-X-C chemokine receptor type 5 (CXCR5)-expressing B cells and T follicular helper cells to the follicle. We here examine the role of CXCL13 and its association with disease in patients with treatment-naïve early rheumatoid arthritis.

Methods

Plasma samples from patients in the OPERA trial were examined for CXCL13 at treatment initiation and after 6 months of treatment with either methotrexate plus placebo (DMARD) (n = 37) or methotrexate plus adalimumab (DMARD + ADA) (n = 39). Treatment outcome was evaluated after 1 and 2 years. CXCL13 plasma levels in healthy volunteers (n = 38) were also examined.

Results

Baseline CXCL13 plasma levels were increased in early rheumatoid arthritis patients in comparison with healthy volunteers. Also, plasma CXCL13 correlated positively with disease activity parameters; swollen joint count 28 (rho = 0.34) and 40 (rho = 0.39), visual analog score (rho = 0.38) and simplified disease activity index (rho = 0.25) (all P <0.05). CXCL13 levels decreased a significantly twofold more in the DMARD + ADA group than in the DMARD group. Baseline CXCL13 plasma levels in the DMARD group correlated inversely with disease activity parameters; disease activity score in 28 joints, four variables, C-reactive protein based (DAS28CRP) (rho = 0.58, P <0.05) at 12 months. High baseline CXCL13 was associated with remission (DAS28CRP less than 2.6) after 2 years.

Conclusions

In treatment-naïve early rheumatoid arthritis patients, plasma CXCL13 levels were associated with joint inflammation. Furthermore, patients with high baseline plasma CXCL13 levels had an improved chance of remission after 2 years. We propose that high CXCL13 concentrations indicate recent onset of inflammation that may respond better to early aggressive treatment. Thus, high levels of CXCL13 could reflect the ‘the window of opportunity’ for optimal treatment effect.

Trial registration

Clinicaltrial.gov NCT00660647. Registered 10 April 2008