Pharmaceutical-grade albumin: impaired drug-binding capacity in vitro

The binding properties of the various albumin solutions were tested in vitro by means of ultrafiltration. Naproxen, warfarin, and digitoxin were used as ligands. HPLC was used to quantitate the total and free drug concentrations. The data were fitted to a model of two classes of binding sites for naproxen and warfarin and one class for digitoxin, using Microsoft Excel and Graphpad Prism.The drugs were highly bound to albumin (95–99.5%). The highest affinity (lowest K1) was found with naproxen. Pharmaceutical-grade albumin solutions displayed significantly lower drug-binding capacity compared to native human serum and Sigma albumin. Thus, the free fraction was considerably higher, approximately 40 times for naproxen and 5 and 2 times for warfarin and digitoxin, respectively. The stabilisers caprylic acid and N-acetyl-DL-tryptophan used in the manufacturing procedure seem to be of importance. Adding the stabilisers to human serum and Sigma albumin reduced the binding affinity whereas charcoal treatment of the pharmaceutical-grade albumin from Octapharma almost restored the specific binding capacity.This in vitro study demonstrates that the specific binding for warfarin and digitoxin is significantly reduced and for naproxen no longer detectable in pharmaceutical-grade albumin. It further shows that the addition of stabilisers may be of major importance for this effect.Albumin is the most abundant protein in blood plasma. One of its main functions is transporting endogenous and exogenous compounds, which might be toxic in the unbound state, but non-toxic as albumin-bound. Thus the albumin may serve as a circulating depot, whereas the unbound substances are the pharmacologically active moieties [1,2]. Changes in the free drug concentration due to displacement or pathological states like impaired renal or liver function and hypoalbuminaemia may increase drug effect, particularly if the drug is highly albumin-bound [3]. Albumin also plays an essential role in generating t