Protein-Drug Binding

Introduction to Protein-Drug Binding

• Macromolecules, such as proteins, DNA, and adipose, interact significantly. Protein-drug binding forms complexes, rendering protein-bound drugs metabolically inactive and pharmacologically inert. Binding occurs intracellularly and extracellularly, represented by the equation: Protein + drug ⇌ Protein-drug complex.

Mechanisms of Protein-Drug Binding

• Drug binding to proteins can be reversible, involving weak bonds that allow equilibrium between bound and unbound states, or irreversible, involving covalent bonds that may cause toxicity (e.g., acetaminophen-induced hepatotoxicity). Most drugs utilize reversible binding.

Classes of Protein-Drug Binding

• The binding of drugs to plasma proteins is reversible.
• Binding of drugs to blood components.
• Plasma protein-drug binding.
• Blood cells drug binding.
• Binding of drug to extravascular tissue protein
• The order of drug affinity to plasma proteins is albumin>à1-acid glycoprotein>lipoproteins>globulins.

Binding of Drugs to Human Serum Albumin

• Albumin, the most abundant plasma protein (59% concentration, 65,000 dalton molecular weight), binds various drugs and endogenous compounds like fatty acids. Drug binding affinity follows the order: Albumin > Glycoprotein > Lipoproteins > Globulins. Site I of Albumin binds NSAIDs and anti-epileptic drugs.
• Site II of human serum albumin, known as the Diazepam binding site, binds benzodiazepines, medium chain fatty acids, ibuprofen, and ketoprofen due to structural affinity. Digitoxin and Tamoxifen bind at Sites III and IV, respectively, with few known drugs.

Binding of Drug to α1-Acid Glycoprotein

• α1-Acid glycoprotein (orosomucoid) has a molecular weight of 44,000.
• The plasma concentration range is 0.04 to 0.1 g%.
• It binds to basic drugs like imipramine, lidocaine, propranolol, and quinidine.

Binding of Drug to Lipoproteins

• Binding with hydrophobic, non-competitive bonds.
• The molecular weight is 2-34 lacks dalton.
• Bound drug dissolves in lipid core, composed of: inside-triglyceride & cholesteryl esters and outside-apoprotein.
• Examples of acidic drugs include Diclofenac.
• An example of a neutral drug is Cyclosporin A.
• An example of a basic drug is Chlorpromazine.

Binding of Drugs to Globulins

• α1 Globulin's synonym is Transcortine/Corticosteroid binding globulin.
• α1 Globulin binds to steroidal drugs, thyroxin, and cyanocobalamine.
• α2 Globulin's synonym is Ceruloplasmine and Vitamin A,D,E,K binds to this.
• β1 Globulin's synonym is Transferin and binds to Ferrous ions.
• β2 Globulin binds to carotenoids.
• γ Globulin binds to antigens.

Drug-Plasma Protein Change Example

• Dexmedetomidine HCl is an alpha 2 adrenergic agonist with sedative and analgesic properties, administered IV.
• Subjects with normal renal function show t1/2 of 136.5 minutes.
• Results for Renally Impaired patients is 113.4 min (P 0.05).

Erythrocytes, or Red Blood Cells

• Red blood cells (RBCs) comprise 45% of blood volume and can bind exogenous and endogenous compounds. Phenytoin, pentobarbital, and amobarbital show a 4:2 RBC/plasma water ratio. RBC drug levels correlate with plasma-free drug concentration; 25% of phenytoin locates in RBCs. Other drugs bind variably to hemoglobin, carbonic anhydrase, and cell membranes.

Binding of Drugs to Extravascular Tissue Protein

• Drug binding to extravascular tissues enhances apparent volume of distribution, site localization, and half-life. Influencing factors include lipophilicity, drug structure, perfusion rate, and pH. Various organs selectively bind specific drugs, impacting their distribution and effects.

Apparent Volume Distribution

• Volume where a drug distributes, with a concentration equal to its plasma concentration.
• It estimates drug distribution extent in the body.
• Apparent volume of distribution is a proportionality constant.
• The constant relates the plasma concentration to the total amount of drug in the body.
• The apparent volume of distribution equation is:
  • XαC
  • X=Vd.C
  • Vd=X/C
• The equation for the volume of distribution for the does of drug given is Vd = Dose of the drug given (Q) / Concentration of drug in plasma (Cp).

Effect of Protein Binding on Apparent Volume of Distribution

• The degree of drug protein binding influences the volume of distribution (VD). Highly bound drugs show lower free fractions and reduced tissue distribution, whereas low-binding drugs have larger free fractions and higher VD. Examples include furosemide and warfarin.

Amount of Drug in the Body Determination

• For a drug in plasma and tissue, the amount bound is given by DB = VpCp + V₁C₁, accounting for both bound and unbound concentrations. At steady state, Cu = Cut; where fu is the unbound drug fraction in plasma and fut in tissue. Replacing C₁ leads to DB = VpCp + V₁ [Cp fu/fut]. Rearranging gives DB/Cp = Vp + V₁ [fu/fut], with Vapp = Vp + V₁ [fu/fut]. This equation connects the drug amount in the body to plasma and tissue volumes and drug fractions. Vapp reflects drug binding and, at steady state, Cu equals Cut, showing no excess distribution volume beyond anatomical limits.

Practice Problem

• Drug A has V app of 20 and another drug, B has a V app of 100 L.
• Drugs have Vp of 4 L and a V t of 10 L, and are 60% bound to plasma protein.
• The free fraction = 0.25 or 25%.
• The fraction of drug A bound to tissues = 1 – 0.25 = 0.75 or 75%.
• tFor drug B, the free fraction is fut= 0.042
• Drugs A & B has 95.8% in the tissues.

Relationship of Plasma Drug-Protein Binding to Distribution and Elimination

• Drugs that are highly bound to plasma proteins have reduced clearance due to impaired hepatocyte entry, decreasing metabolism. Increased protein binding also prolongs elimination half-lives, particularly for cephalosporins.

Relationship Between VD and Drug Elimination Half-Life

• Drug elimination occurs via renal and metabolic pathways. Extensive distribution dilutes drugs, complicating kidney filtration. Increased clearance extends half-life, influenced by lipophilicity, tissue protein binding, and low plasma protein binding.

Drug with a Large Volume of Distribution and a Long Elimination

• The macrolide antibiotic dirithromycin distributes extensively with a steady-state volume of about 800 L and a 44-hour elimination half-life.

Factors affecting Protein binding of drugs

• Increase in lipophilicity enhances drug binding; for example, cloxacillin is 95% bound compared to ampicillin's 20%. Hypoalbuminemia impairs protein-drug binding.

Concentration of Drug in the Body

• At low concentrations, drugs bind to proteins, while higher concentrations lead to free drugs. Lidocaine prefers AAG over HSA; digoxin binds more to cardiac muscle proteins.

Drug Interactions.

• Displacement interactions occur when drugs like warfarin and phenylbutazone compete for binding sites. A displacing drug raises plasma concentration and binds available proteins, affecting therapeutic outcomes.

Allosteric Changes in Protein Molecule.

• Alteration of drugs and metabolites impacts binding; aspirin modifies NSAIDs and oxygen binds hemoglobin..

Protein Related Factors in Protein Drug-Binding

• Disease states like renal failure reduce albumin synthesis, affecting acidic drug binding, while neutral and basic drugs remain stable. Hepatic failure affects binding based on AAG levels, and inflammatory conditions elevate AAG, influencing basic drug binding. Indomethacin binds three sites on albumin, whereas lidocaine binds two sites on AAG with HSA.

Patient Related Factors

• Age: Neonates with low albumin content, result in more free drug.
• Infants that a young are high dose of Digoxin due to the renal. -Albumin is lower in the eldery which result in free drug

Protein-bound drug properties

• Elimination occurs with unbound drugs; only the unbound fraction is metabolized, e.g., phenylbutazone.

Clinical Significance of Drug-Protein Binding

• Hypoalbuminemia can severally impair protein-drug bindings Factors that decrase levels of (1) Protein synthesis, (2) protein catabolism, (3) Distribution of albumin to extravascular and extravascular (4) Excercise elimination of the protein And more

Clinical Examples of Drug Protein Binding

• The macrolide antibiotic dirithromycin is extensively distributed in the tissues, resulting in a large steady state in the volumes Is higher since it goes in cells