Effects of serum albumin on the photophysical characteristics of synthetic and endogenous protoporphyrin IX

The study of the interaction of synthetic protoporphyrin IX (PpIXs) and protoporphyrin IX extracted from Harderian glands of ssp Rattus novergicus albinus rats (PpIXe) with bovine serum albumin (BSA) was conducted in water at pH 7.3 and pH 4.5 by optical absorption and fluorescence spectroscopies. PpIXs is present as H- and J-aggregates in equilibrium with themselves and with monomers. The PpIXs charge is 2− at pH 7.3 and 1− at pH 4.5. This increases its aggregation at pH 4.5 and shifts the equilibrium in favor of J-aggregates. In spite of electrostatic attraction at pH 4.5, where BSA is positive, the binding constant (Kb) of PpIXs to BSA is 20% less than that at pH 7.3, where BSA is negative. This occurs because higher aggregation of PpIXs at pH 4.5 reduces the observed Kb value. At both pHs, water-soluble PpIXe exists in the monomeric form with the charge of 1− and its Kb exceeds that of PpIXs. At pH 4.5, its Kb is 12 times higher than that at pH 7.3 due to electrostatic attraction between the positively charged BSA and the negatively charged PpIXe. The higher probability of PpIXe binding to BSA makes PpIXe more promising as a fluorescence probe for fluorescence diagnostics and as a photosensitizer for photodynamic therapy. The existence of PpIXe in the monomeric form can explain its faster cell internalization. Aggregation reduces quantum yields and lifetimes of the PpIXs excited states, which explains higher phototoxicity of PpIXe toward malignant cells compared with PpIXs.

Effect of pH on weakly acidic and basic model drugs and determination of their ex vivo transdermal permeation routes

ABSTRACT The aim of the present study was to investigate the effect of donor pH on the transdermal permeability of the model drugs across rat skin and also to determine the major route of transport of the drugs. Weakly acidic drugs (partition coefficient) ibuprofen (3.6), aceclofenac (3.9), glipizide (1.9) and weakly basic drugs olanzapine (3.6), telmisartan (6.0), and sildenafil citrate (1.9) were selected for the study. The ex vivo permeation studies of these drugs at different donor pH (pH - 1.2, 4, 5, 6.8, 7.4, and 8) using Franz diffusion cell (area, 7.54 cm2) has shown a pH-dependent permeability. Among these drugs the weakly acidic drugs has shown higher permeation rates compared to the weakly basic drugs. The permeability coefficient and the distribution coefficient of the weakly basic drugs increased on increasing the pH whereas the weakly acidic drugs showed an inverse relation. The weakly basic drugs also showed an increase in permeation with increase in the fraction of unionized species indicating dominance of transcellular route of permeation. With an exception of sildenafil citrate, a weakly basic salt form of the drug which showed a high permeation value at pH 7.4 where 57% of the drug was unionized, indicating the involvement of both paracellular and transcellular route in its permeation.