*Purpose: At present, more older and higher risk donor kidneys are accepted resulting in increased uncertainty about quality. This may result in unjustified discard of donor organs. To increase organ utilisation without compromising outcomes, assessment of real-time perfusion using near-infrared fluorescence (NIRF) imaging prior to transplantation may assist in the decision making. ZW800-1 is a clinical applied non-toxic NIRF dye, which is rapidly and efficiently cleared by the kidney, thus allowing immediate functional testing. Its ability to visualise (cortical) kidney perfusion may be more reliable than the experienced surgical eye. In this pilot, we studied the feasibility of NIRF imaging as a technique to measure perfusion and kidney function during normothermic machine perfusion (NMP).

*Methods: Slaughterhouse pig kidneys were flushed, underwent 2h hypothermic machine perfusion, and were placed on NMP at 37°C for 7h with oxygenated, leukocyte-depleted autologous whole blood. Dose-escalation experiments (0.125; 0.25; 1.0; 4.0 mg/kg per kidney weight) were conducted to obtain robust and reproducible images. Boluses of ZW800-1 were injected intravenously after 1h and 6h of NMP. Following ZW800-1 injection fluorescent images of kidneys were quantified as signal-to-background ratios (SBRs) using the FLARE imaging system. Urine and perfusate samples were collected to measure ZW800-1 concentration and calculate excretion as a reflection of kidney function.

*Results: A series of dosage experiments showed that 1.0 mg/kg of the compound was optimal, allowing reliable assessment of perfusion with a clear differentiation between well perfused and marginally perfused kidneys or areas of the kidneys. The average SBR (n=5) in the 1.0 mg/kg group decreased from 3.42±1.09 to 2.28±0.73, corresponding with a ZW800-1 concentration in the perfusate decreasing from 100±51 µg/ml up to 6.4±36.5 µg/ml, whilst increasing in the urine up to 8.7±14.4 µg/ml throughout the perfusion. The clearance of dye per kidney (median 17%±24%) was directly associated with diminished fluorescence intensity. In kidneys without any urine production, the SBR remained the same.

*Conclusions: This pilot study shows that NIRF imaging is feasible during NMP. By assessing the fluorescent intensity of different areas of the kidney and the urine dye excretion, the application of NIRF imaging could provide clinically relevant information concerning perfusion and function, potentially helping the clinical decision in accepting a high risk donor kidney.

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