Protein-bound Concentration Research Articles

The free hormone hypothesis: Correlation of decreases in PSA with free testosterone rather than total testosterone in men with advanced prostate cancer treated with GTx-758.

e16015 Background: Androgen deprivation therapy (ADT) improves disease-free survival but disease progression is related, in part, to ineffective castration. The free hormone hypothesis states that the biological activity of steroid hormones is affected by its unbound (free) rather than its protein-bound concentration. Serum total testosterone (T) concentrations predominantly reflect the T bound to plasma proteins and do not accurately predict prostatic levels of T. Methods: In a Phase II study (G200705), men with advanced prostate cancer (n=159) were randomized to receive 1000 mg or 2000 mg GTx-758 daily or leuprolide as their initial ADT. Serum total T (mass spectrometry), free T (equilibrium dialysis), SHBG and PSA concentrations were measured. A second Phase II study (G20007) was performed in men (n=9) with CRPC who then received GTx-758 2000 mg daily. Results: Although both treatments reduced serum total T levels to < 50 ng/dL, leuprolide decreased them to a greater extent. However, GTx-758 caused greater reductions in serum PSA, suggesting that total T concentrations did not accurately reflect the suppression of androgen activity. Both dosages of GTx-758 reduced free T levels to a greater extent (mean of 0.7 and 0.4 pg/ml at day 60, and 0.4 and 0.4 pg/ml on day 90, respectively) than leuprolide (mean of 1.4 pg/ml on day 60 and 1.4 pg/ml on day 90; p values <0.03). Similar clinical results were observed in CRPC patients where GTx-758 daily resulted in a 71% decrease in %free T and clinically relevant PSA reductions in men maintained on ADT with LHRH agonists. As a result of adverse events at higher doses of GTx-758, the trial was stopped early. Conclusions: The ERα agonist, GTx-758, reduced the biologically active form of T, free T, to significantly lower levels than leuprolide. Reductions in PSA appeared to be more highly associated with changes in free T. These data provide compelling evidence to support the free hormone hypothesis and suggest that serum free T concentrations would provide a better measure of therapeutic efficacy in ADT than total T. A Phase II clinical trial utilizing lower doses of GTx-758 (G200712) is currently being performed. Clinical trial information: NCT01326312.

Fluorescent Fatty Acid Transfer from Bovine Serum Albumin to Phospholipid Vesicles: Collision or Diffusion Mediated Uptake

The extent of palmitate uptake by hepatocytes is dependent upon the surface charge of the extracellular binding protein. Specifically, hepatocyte uptake is greater when palmitate is bound to cationic binding proteins than when it is bound to anionic proteins. To further understand the role of protein surface charge on the uptake process of protein-bound ligands, we examined the rate of transfer of fluorescent anthroyloxy palmitic acid (AOPA) in the presence of anionic and cationic extracellular proteins to model membranes containing different surface charged groups. AOPA transfer rate in the presence of bovine serum albumin (ALB; isoelectric point pI = 4.8-5.0) or modified ALB (ALBe; pI = 7.0-7.5) to negative, positive and neutral lipid vesicles was investigated using a fluorescence resonance energy transfer assay. The rate of AOPA transfer from both proteins was decreased when ionic strength was increased; directly dependent on the concentration of acceptor lipid vesicles; and was affected by both the lipid membrane surface charge and protein-bound concentration. The data support the notion that AOPA transfer from binding proteins to lipid membranes occurred through two concomitant processes, aqueous diffusion of the unbound ligand (diffusion-mediated process) and a collisional interaction between the protein-ligand complex and acceptor membrane. The contribution of diffusional mediated transfer to the overall uptake process was determined to be 3 to 4 times less than the contribution of a collisional interaction. This study strengthened the hypothesis that charged amino acid residues on proteins are important for effective collisional interaction between protein-ligand complexes and cell membranes through which more free ligand could be supplied for the uptake process.

The free hormone hypothesis: a physiologically based mathematical model.

The free hormone hypothesis states that the biological activity of a given hormone is affected by its unbound (free) rather than protein-bound concentration in the plasma. The fundamental mathematical and physiological principles relating to this hypothesis are reviewed, along with experimental data that shed light on its validity. It is shown that whether or not this hypothesis is likely to be valid for any given hormone will depend largely on which step in the tissue uptake process (plasma flow, dissociation from plasma binding proteins, influx, or intracellular elimination) is rate-limiting to the net tissue uptake of that hormone. It is further shown that the free hormone hypothesis could hold even if tissue uptake of hormone occurred by a mechanism that acted directly on one or more circulating protein-bound pools of hormone. Indeed, many of the data previously interpreted as being inconsistent with the free hormone hypothesis are in fact readily consistent with it when its predictions are fully understood. Nevertheless, the free hormone hypothesis is not likely to be valid for all hormones with respect to all tissues. It is likely to be valid with respect to all tissues for the thyroid hormones, for cortisol, and for the hydroxylated metabolites of vitamin D. For many of the other steroid hormones, however, it is likely to be valid with respect to some tissues, but not with respect to others (in particular, the liver). And for some of the steroid hormones (in particular, progesterone) it may not hold at all.