OFM recirculation and OFM suction: Advanced in-vivo open flow microperfusion methods for direct and absolute quantification of albumin in interstitial fluid

Objective: To implement OFM-recirculation and OFM-suction capable of direct and absolute in-vivo quantification of albumin in the ISF of pigs. Approach: OFM-recirculation and OFM-suction were used to collect ISF in-vivo in pigs and lymph was collected from the same pigs after the OFM sampling. Blood was collected before and after the OFM-sampling, plasma was isolated and mean albumin plasma concentrations per pig were used to yield albumin ISF-to-plasma ratios. We characterized the quality of the collected undiluted ISF via (1) stable albumin ISF-to-plasma ratio in OFM-recirculation and in OFM-suction samples, (2) comparison of albumin ISF-to-plasma ratios from OFM-recirculation and OFM-suction and (3) comparison of normalized albumin concentrations in the ISF and lymph. Main results: Both advanced OFM methods were successfully implemented and albumin was quantified from the collected ISF samples. OFM-recirculation reached stable albumin ISF-to-plasma ratios after 20 recirculation cycles. Absolute ISF albumin concentrations were 11.2 mg/ml (OFM-recirculation) and 14.2 mg/ml (OFM-suction). Albumin ISF-to-plasma ratios were 0.39 ± 0.04 (OFM -recirculation) and 0.47 ± 0.1 (OFM-suction). Significance: Knowledge of the ISF protein content is of major importance when assessing PK/PD effects, especially of highly protein bound drugs. Up to now, only the albumin blood values have been available to determine the degree of protein binding in several tissues. OFM recirculation and OFM suction allow direct, absolute quantification of albumin in ISF for the first time and enable investigation of the degree of protein binding of a drug directly in its target tissue