Biopharmaceutical Barriers to Drug Action - Solubility and Cell Permeability Handout.pptx

Biopharmaceutical Barriers to Drug Action: Solubility and Cell Permeability     Background and BCS Biopharmaceutical Classification System Solubility Cell Monolayer Permeability Topic’s Objectives  Understand the role of ADME in drug bioavailability and the Biopharmaceutics Classification Sy...

Biopharmaceutical Barriers to Drug Action: Solubility and Cell Permeability     Background and BCS Biopharmaceutical Classification System Solubility Cell Monolayer Permeability Topic’s Objectives  Understand the role of ADME in drug bioavailability and the Biopharmaceutics Classification System  Understand the role of solubility in oral bioavailability  Describe the experimental setup for cell monolayer permeability Drug Journey to Target  For a drug to exhibit a therapeutic effect, its molecules must reach the target in the body, and stay there for a proper time and in sufficient concentration  The processes the drug molecules undergo in the body represent pharmacokinetics and can be classified as (ADME):  Absorption - from the administration site to the bloodstream  Distribution – from the bloodstream to tissues  Metabolism – enzymatic and spontaneous (e.g., hydrolysis)  Excretion – into urine, sweat, expired air, feces…  Further classification terms for these processes are:  Elimination = metabolism + excretion  Disposition = distribution + elimination  Ability of a drug to overcome barriers involved in the PK processes determines its overall exposure and bioavailability. Drug Fates & Membrane Transport  Absorption: GI and pulmonary tracts, skin, mucose, cornea  In the bloodstream: convection and diffusion, binding to formed particles (erythrocytes, leukocytes, platelets), and (lipo)proteins  Distribution: tight and leaky capillaries, transport into the extracellular space and into cells in tissues  Metabolism: bilayer buildup in some cases  Excretion: filtration, secretion, reabsorption Underlining: trans-cellular transport involved paracellular transport involved Drug Diffusion in the Body: Mechanisms enteral • oral • buccal • sublingual • peroral • rectal… parenteral • intravenous injection/infusion • intraarterial injection… parenteral • subcutaneous • intramuscular… percutaneous • topical • nasal • vaginal… bloodstream • mostly passive • trans-bilayer diffusion • occasionally aided by • active transport or endo-/exocytosis • occasionally hindered by efflux • distribution ADMINISTRATION absorption ADMINISTRATION mostly passive trans-bilayer diffusion diffusion into extracellular fluid of organ with porous capillary walls occasional mechanisms as in absorptio Drug Accumulation in the Body Parts PHASES plasma • albumin • 1-glycoprotein • transthyretin • lipoproteins aqueous • neutral • acidic - lysosomes - hypoxic cancer cells - urine • basic - mitochondria lymph • albumin • chylomicra extracellular • albumin • ECM components intracellular • actin • saturable • competitive • structure-specific solvation binding MACROMOLECULES lipoid • phospholipids • triglycerides • lipoprotein particles • linear • noncompetitive • independent of detailed structure Barriers for Orally Administered Drug An oral drug must be able to:  Dissolve to an acceptable extent  Survive a range of pHs  Cross membranes in villi and capillary walls  Avoid excessive binding to plasma proteins and lipoproteins  Survive liver metabolism  Avoid active transport to bile  Cross membranes of capillaries and tissue cells  Partition into the target organ  Avoid partition into Desirable Characteristics of Oral Drug Besides stability and proper protein binding, the minimum requirements for oral bioavailability are:  Solubility in water to have a sufficient free concentration  in the stomach and GI juice for GI absorption  in the bloodstream, tissues, and organs to accomplish distribution (the human body is 50-80% water)  Permeability for the membranes mostly by passive transbilayer diffusion, for which the drug needs  appropriate lipophilicity and hydrophilicity balance • too hydrophilic drugs do not enter bilayers • too lipophilic drugs do not leave bilayers • only drugs with optimal balance pass through several bilayers as required for absorption and distribution  appropriate ionization – ionized molecules do not pass Biopharmaceutic Classification System  Importance of permeability and aqueous solubility of drug substance (including the rate of dissolution of the drug product) for oral bioavailability is underlined by BCS  BCS is a framework which provides assurance of in vivo bioequivalence based on extensive in vitro characterization  Class I drug candidates can obtain FDA biowaivers reducing human testing  Class II is subdivided for low dissolution rate (IIa) and low product (IIb) solubility Solubility  Solubility S is defined as the concentration (mol/L) of a drug (solid, liquid, or gas) in a saturated solution, at given temperature and pressure. Historically, and for practical reasons, S is often given as % w/w or % w/v.  Saturated solution of a solid/liquid/gaseous drug is in equilibrium with the solid/liquid/gas phase:  the solvent is not capable of dissolving more drug under the given conditions  the dissolved drug concentration has reached the maximum and does not increase anymore  For drugs, the most interesting solubilities are in water and lipoid phases (bilayer cores of membranes, lipoproteins, interiors of globular proteins) of the body Descriptive Solubility Terms (USP) Term Parts of solvent required to dissolve 1 part of solute Approx. S Very soluble <1 [% w/w] >50 Freely soluble 1 – 10 10-50 Soluble 10 – 30 3-10 Sparingly soluble 30 – 100 1-3 Slightly soluble 100 –1000 0.1-1 Very slightly soluble 1000 – 10,000 0.01-0.1 (Practically) insoluble > 10,000 <0.01 Factors Determining Solubility Solubility is given by energy of solution, which includes three components (the overall sum must be a gain):  Energy to bring the drug molecule out of the pure drug phase (loss)  for gaseous and liquid drugs – usually small  for solid drugs – can be significant, the higher the energies, the higher the melting points of crystals  Energy to create a cavity in the solvent (loss)  significant only in water because of H-bonds  increases with the size of the drug molecule  Energy of solute-solvent interactions (gain) – significant in water, low in lipoid phases - solvation Solvation / Hydration … (step 3) is determined by the interactions of the drug molecules with the molecules of water or lipoid phases in a solution The drug-solvent interactions are weak, mostly attractive interactions, which may include  electrostatic, H-bonding, dispersion, and hydrophobic interactions in water  only dispersion interactions in lipoid phases The solvation is strongest between similar drug and solvent molecules  the “like dissolves like” principle Polymorphism The same compound crystallizes in different crystal forms structure and molecular conformations of two polymorphs of spiperone Solubility of Polymorphs  Polymorphs differ in the energy of release from the solute phase  they often have different solubility  Metastable polymorphs (less stable forms) usually have higher solubility than more stable forms because the release from the solid form requires less energy and cavity and solvation term are independent of the solid form  Amorphous powders lack the crystalline structure and have higher solubility than crystalline forms  Hydrated crystals (solvates of water): tend to exhibit lower aqueous solubility than their anhydrate forms. Dissolution Solid forms of drugs are often available in several forms  crystalline forms  polymorphs (more than one crystalline form)  amorphous forms If feasible, amorphous form is chosen for a solid dosage form because of highest solubility Dissolution is often the rate limiting step in getting the drug into systemic circulation after oral administration Dissolution rate is a critical parameter for solid dosage forms, especially for oral administration Dissolution Testing  USP provides several official methods of testing differing in the setup of the dissolution apparatus that is used to reduce the hydration layer  rotating basket  paddle  reciprocating cylinder  flow-through cell  The dissolution medium may imitate the content of GI tract where the formulation is expected to dissolve (0.1 M HCl, simulated gastric or intestinal juice), temperature 37  C  The samples of the medium are taken at appropriate times, the solid particles are removed, the liquid is analyzed for drug content, and the data are processed by a kinetic model. Dissolution Testing I USP Apparatus 1  rotating basket (100 rpm) USP Apparatus 2  paddle (50 rpm) USP Apparatus 3  reciprocating cylinder USP Apparatus 4  flow-through cell Dissolution Testing II Automated tablet drop The process starts at the same moment in all vessels Continuous or discrete sampling of the media Strategies to Improve Drug Solubility  Chemical structure modifications – only in early stages of drug development  Modification of physical properties of solid dosage forms  utilization of polymorphism or amorphous state  micronization: break down the crystal lattice of the solid (it is more than just reducing the particle size)  salt formation  Formulation of solvent system (solution dosage forms)  adjust pH (use buffer systems): keep drug in ionized state  use appropriate cosolvents (i.e. alcohol, hydrogen bonding organic solvents)  + surfactants (as excipients)  + complexation: association between two or more molecules to form a coordination complex Cell Monolayer Permeability Setup (with microvilli) Cell Monolayers  Cell lines forming confluent monolayer AP of polarized cells, i.e., having different apical (AP) and basolateral (BL) sides BL  Caco-2 cells is a human colon epithelial cancer cell line, gold standard for intestinal absorption simulations:  differentiated and polarized with intercellular tight junctions  a well-differentiated brush border  typical small-intestinal nutrient transporters  resembling the enterocytes lining the small intestine  Madin-Darby canine kidney (MDCK) cells are susceptible to viral infection, including influenza serotypes  development of new influenza vaccine candidates for humans  identification of P-glycoprotein substrates and inhibitors Cell Monolayers Permeability Data  The cellular uptake of tested compounds is also measured  Equal transport rates in both directions (APBL and BLAP) indicate the only passive trans-bilayer transport is involved.  Involvement of transporters can also be checked using transporter inhibition and only measurement in the absorptive (APBL) direction  the result is the intrinsic permeability coefficient  comparison with the apparent permeability coefficient (without inhibition) informs about the efflux or influx Mol. Pharmaceutics 2017, 14, 5, 1601–1609