For about half a century, the binding of drugs to plasma albumin, the “silent receptor,” has been recognized as one of the major determinants of drug action, distribution, and disposition. In the last decade, the binding of drugs, especially but not exclusively basic entities, to another plasma protein, alpha 1-acid glycoprotein (AAG), has increasingly become important in this regard. The present review points out that hundreds of drugs with diverse structures bind to this glycoprotein. Although plasma concentration of AAG is much lower than that of albumin, AAG can become the major drug binding macromolecule in plasma with significant clinical implications. Also, briefly reviewed are the physiological, pathological, and genetic factors that influence binding, the role of AAG in drug-drug interactions, especially the displacement of drugs and endogenous substances from AAG binding sites, and pharmacokinetic and clinical consequences of such interactions. It can be predicted that in the fulture, rapid auto...

Human alpha-1-glycoprotein and its interactions with drugs

Purpose: In a phase I trial for patients with refractory solid tumors, hedgehog pathway inhibitor vismodegib (GDC-0449) showed little decline in plasma concentrations over 7 days after a single oral dose and nonlinearity with respect to dose and time after single and multiple dosing. We studied the role of GDC-0449 binding to plasma protein alpha-1-acid glycoprotein (AAG) to better understand these unusual pharmacokinetics. Experimental Design: Sixty-eight patients received GDC-0449 at 150 ( n = 41), 270 ( n = 23), or 540 ( n = 4) mg/d, with pharmacokinetic (PK) sampling at multiple time points. Total and unbound (dialyzed) GDC-0449 plasma concentrations were assessed by liquid chromatography/tandem mass spectrometry, binding kinetics by surface plasmon resonance–based microsensor, and AAG levels by ELISA. Results: A linear relationship between total GDC-0449 and AAG plasma concentrations was observed across dose groups ( R 2 = 0.73). In several patients, GDC-0449 levels varied with fluctuations in AAG levels over time. Steady-state, unbound GDC-0449 levels were less than 1% of total, independent of dose or total plasma concentration. In vitro , GDC-0449 binds AAG strongly and reversibly ( K D = 13 μmol/L) and human serum albumin less strongly ( K D = 120 μmol/L). Simulations from a derived mechanistic PK model suggest that GDC-0449 pharmacokinetics are mediated by AAG binding, solubility-limited absorption, and slow metabolic elimination. Conclusions: GDC-0449 levels strongly correlated with AAG levels, showing parallel fluctuations of AAG and total drug over time and consistently low, unbound drug levels, different from previously reported AAG-binding drugs. This PK profile is due to high-affinity, reversible binding to AAG and binding to albumin, in addition to solubility-limited absorption and slow metabolic elimination properties. Clin Cancer Res; 17(8); 2512–20. ©2011 AACR .

https://clincancerres.aacrjournals.org/content/clincanres/17/8/2512.full.pdf

Pharmacokinetics of Hedgehog Pathway Inhibitor Vismodegib (GDC-0449) in Patients with Locally Advanced or Metastatic Solid Tumors: the Role of Alpha-1-Acid Glycoprotein Binding

Identification and characterization of biomarkers in body fluids such as serum or urine serve as a basis for early detection of diseases, particularly of cancer. Performing 2-DE with subsequent MS analyses, conventional immunoblotting and immunohistochemistry we identified two proteins, orosomucoid (ORM) and human zinc-alpha(2)-glycoprotein (ZAG), which were increased in the urine samples of patients with bladder cancer in comparison to the urine samples of healthy volunteers. The highest amount of both proteins was found in invasive bladder cancer stages such as pT2-3. Immunohistochemical studies showed ORM in inflammatory cells but also in endothelial cells of blood vessels within or adjacent to the tumor area and in part of the tumor cells. ZAG was prominent in tumor cells at the tumor invasion front. Additionally, ZAG was localized at the luminal surface of normal urothelium, which switches to the basal side when a superficial papillary tumor was observed. These results show that we have been able to identify two new proteins that may be related to the development of superficial bladder cancer and to its switch to an invasive phenotype.

Stage‐dependent increase of orosomucoid and zinc‐alpha2‐glycoprotein in urinary bladder cancer

Immobilization of α1-acid glycoprotein for chromatographic studies of drug-protein binding

Objective

Our objective was to evaluate the association between exposure to unbound docetaxel and neutropenia in patients with cancer and to identify factors influencing unbound docetaxel clearance.



Methods

Docetaxel was administered once every 3 weeks at a dose of 75 mg/m2 to 49 patients with normal liver function (n = 40, group 1) or mild elevations in liver function test results (n = 9, group 2) or at a dose of 50 mg/m2 to patients with moderate elevations in liver function test results (n = 6, group 3). Pharmacokinetic studies and toxicity assessments were performed during the first cycle of therapy. Total docetaxel concentrations were determined by HPLC and tandem mass spectrometry, and unbound docetaxel fraction was determined by equilibrium dialysis.



Results

In patients with normal liver function, unbound docetaxel disposition was characterized by mean (±SD) maximum plasma concentration (Cmax), area under the curve (AUC), and clearance values of 233 ± 101 ng/mL, 32 ± 143 ng/mL · h, and 565 ± 329 L/h, respectively. Unbound clearance varied 8.5-fold; polysorbate 80 exhibited mean (±SD) Cmax, AUC, and clearance values of 451 ± 221 μg/mL, 528 ± 217 μg/mL · h, and 8.18 ± 3.66 L/h, respectively; and clearance varied 6.7-fold. Unbound docetaxel clearance was reduced in patients with moderate liver impairment (groups 1 and 2 versus group 3, P = .020). From multiple linear regression analysis, only polysorbate 80 AUC and liver impairment were significantly associated with unbound docetaxel clearance. Both unbound docetaxel AUC and total AUC were correlated with the percentage decrements in absolute neutrophil count (P = .002 and P = .029, respectively), as well as the worst grade of neutropenia (P = .013 and P = .220, respectively), where higher exposure was associated with worse hematologic toxicity.



Conclusions

Exposure to unbound docetaxel is closely related to drug-induced hematologic toxicity and should be considered in future pharmacologic investigations.



Clinical Pharmacology & Therapeutics (2005) 77, 43–53; doi: 10.1016/j.clpt.2004.09.005

Relationship of systemic exposure to unbound docetaxel and neutropenia.

Expression of the genetic variants of human alpha-1-acid glycoprotein in cancer.

Aims The plasma binding of the cyclosporin D analogue SDZ PSC 833 was investigated in vitro.



Methods The plasma total binding constant (corresponding to the bound-to-free concentration or binding ratio) was determined at 37° C by the erythrocyte partitioning technique on plasma samples from three healthy volunteers and three cancer patients. Lipoproteins were also removed from plasma samples from three healthy volunteers by a standard ultracentrifugal technique.



Results SDZ PSC 833 plasma binding was 97.8±1.1% and 97.3±0.2% in samples from three healthy volunteers and three cancer patients respectively. More than 95% of blood SDZ PSC 833 was distributed in plasma. When the original plasma samples of three individuals were delipidated, SDZ PSC 833 binding was strongly decreased (58% bound to plasma proteins) and when lipoproteins were resuspended in the delipidated plasma samples to produce varying lipoprotein plasma concentrations, the binding increased continuously with the fraction of added lipoproteins. When lipoproteins were resuspended to restore the original lipoprotein plasma content, the % plasma-bound SDZ PSC 833 increased to 98.2%, close to the value observed with the original plasma (98.7%).



Conclusions These results clearly indicate that SDZ PSC 833 plasma binding is mainly determined by lipoproteins and that in blood, most of SDZ PSC 833 is distributed in plasma.

https://bpspubs.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2125.1998.00663.x

Emmanuelle Malaurie

Papers

Expression of the genetic variants of human alpha-1-acid glycoprotein in cancer.

Role of lipoproteins in the plasma binding of SDZ PSC 833, a novel multidrug resistance-reversing cyclosporin.