Octanol / Water Partitioning Coefficient logp and ClogP

Transcription

1 Lipid solubility and the brain uptake of compounds Higher the Kow, the greater the brain uptake Specific carriers can facilitate brain uptake (glucose, lucine, dopa) Binding to plasma proteins can reduce uptake (phenytoin) Octanol / Water Partitioning Coefficient logp and ClogP The logp is an important physicochemical parameter for oral absorption Measure for the lipophilicity of a compound. ClogP is the calculated log octanol/water partition coefficient, It relates to solubility and influences the ability of a compound to permeate cell membranes Too hydrophilic compounds (negative logp) cannot permeate membranes Too lipophilic compounds (high logp) are insoluble, poorly permeate through membranes (get stuck in the lipophilic bilayer). 1

2 Predicting Favorable ADME in Drug Development - Lipinski s Rule of Five Lipinski's rule: An orally active drug has no more than one violation of the following : 1. No more than 5 hydrogen bond donors (N or O with >1 H atom) 2. Not more than 10 hydrogen bond acceptors (N or O atoms) 3. A molecular mass < 500 daltons 4. An octanol-water partition coefficient (log P) < 5 A rule of thumb to evaluate: If a chemical compound with a pharmacological /biological activity has properties that may make it a likely orally active drug. Rule formulated by Chris Lipinski in 1997, based on the observation that most medication drugs are relatively small and lipophilic molecules * As with all rules, many exceptions! Predicts that poor absorption and permeability of potential drug candidates will occur if: 1) Number of H-bond donors > 5 2) Number of H-bond acceptors > 10 3) Molecular Weight > 500 4) ClogP > 5 Lipinski s rule does not predict if a compound is pharmacologically active. Rule is important to keep in mind during drug discovery when a lead structure is optimized step-wise to increase the activity and selectivity. Candidate drugs that conform to the RO5 tend to have lower attrition rates during clinical trials Drug Metabolism / Biotransformation Learning objectives Distinguish the differences between Phase I and Phase II reactions Recognize the enzymes involved in these processes and that various factors (age, genetic predisposition/polymorphisms, co-exposure to other drugs, diet, illness) may affect these processes Be familiar with at least one example of one of the following types of Phase I biotransformation reactions: Oxidation, Reduction, Hydrolysis. The same goes for the following types of Phase II biotransformation reactions: Conjugation with glucuronide, glutathione, sulfate. Recognize that one of the possible consequences of biotransformation is the conversion of prodrugs to active drug, or active drug to reactive intermediates, and that depending upon their chemical nature, these intermediates can modify molecular targets through different mechanisms 2

3 Drug Metabolizing Enzyme System Phase I and Phase II Biotransformation Phase I metabolism Functionalization Addition or exposing of a reactive functional group such as -OH, -SH, -NH2 or COOH In general, Phase I metabolism prepares the xenobiotic for subsequent Phase II reactions Phase II Reactions Enzymatic reactions that conjugate large watersoluble, charged (polar) biomolecules to xenobiotics The Truck-Hitch-Trailer Analogy to Xenobiotic Biotransformation Foreign Chemical (xenobiotic) Phase 1 enzymes add or expose a functional group Phase 2 enzymes conjugate (transfer) endogenous molecules* to the functional group lipophilic not charged not water soluble poorly excretable * sugars, amino acids, sulfates, acetyl groups HITCH Phase 1 enzymes add or expose a functional group still lipophilic possibly reactive poorly water soluble poorly excretable catalyzed by P450s TRAILER not lipophilic usually not reactive water soluble products Excretable catalyzed by transferases 3

4 Biotransformation Enzyme-Containing Cells in Various Organs Organ Cell(s). Liver Parenchymal cells (hepatocytes) Kidney Proximal tubular cells (S3 segment) Lung Clara cells, Type II alveolar cells Intestine Mucosa lining cells Skin Epithelial cells Testes Seminiferous tubules, Sertoli cells Mixed Function Oxidase System (MFO) (Phase I Metabolism) MFOs Contain: Cyt. P450, 2 flavoproteins (P450 reductases), Cytochrome b5 4

5 CYP families in humans Humans have 57 genes and more than 59 pseudogenes divided among: 18 families of cytochrome P450 genes 43 subfamilies Function Family Members Names CYP1 drug and steroid (especially estrogen) metabolism 3 subfamilies, 3 genes, 1 pseudogene CYP1A1, CYP1A2, CYP1B1 CYP2 drug and steroid metabolism CYP2A6, CYP2A7, CYP2A13, CYP2B6, CYP2C8, CYP2C9, CYP2C18, CYP2C19, 13 subfamilies, 16 genes, 16 pseudogenes CYP2D6, CYP2E1, CYP2F1, CYP2J2, CYP2R1, CYP2S1, CYP2U1, CYP2W1 CYP3 drug and steroid (including testosterone) metabolism 1 subfamily, 4 genes, 2 pseudogenes CYP3A4, CYP3A5, CYP3A7, CYP3A43 CYP4 arachidonic acid or fatty acid metabolism 6 subfamilies, 11 genes, 10 pseudogenes CYP4A11, CYP4A22, CYP4B1, CYP4F2, CYP4F3, CYP4F8, CYP4F11, CYP4F12, CYP4F22, CYP4V2, CYP4X1, CYP4Z1 CYP5 thromboxane A 2 synthase 1 subfamily, 1 gene CYP5A1 CYP7 bile acid biosynthesis 7-alpha hydroxylase of steroid nucleus 2 subfamilies, 2 genes CYP7A1, CYP7B1 CYP8 varied 2 subfamilies, 2 genes CYP8A1 (prostacyclin synthase), CYP8B1 (bile acid biosynthesis) CYP11 steroid biosynthesis 2 subfamilies, 3 genes CYP11A1, CYP11B1, CYP11B2 CYP17 steroid biosynthesis, 17-alpha hydroxylase 1 subfamily, 1 gene CYP17A1 CYP19 steroid biosynthesis: aromatase synthesizes estrogen 1 subfamily, 1 gene CYP19A1 CYP20 unknown function 1 subfamily, 1 gene CYP20A1 CYP21 steroid biosynthesis 2 subfamilies, 2 genes, 1 pseudogene CYP21A2 CYP24 vitamin D degradation 1 subfamily, 1 gene CYP24A1 CYP26 retinoic acid hydroxylase 3 subfamilies, 3 genes CYP26A1, CYP26B1, CYP26C1 CYP27 varied 3 subfamilies, 3 genes CYP27A1 (bile acid biosynthesis), CYP27B1 (vitamin D3 1-alpha hydroxylase, activates vitamin D3), CYP27C1 (unknown function) CYP39 7-alpha hydroxylation of 24-hydroxycholesterol 1 subfamily, 1 gene CYP39A1 CYP46 cholesterol 24-hydroxylase 1 subfamily, 1 gene CYP46A1 CYP51 cholesterol biosynthesis 1 subfamily, 1 gene, 3 pseudogenes CYP51A1 (lanosterol 14-alpha demethylase) Cytochrome P450 Characteristics Broad substrate specificity - Most drug metabolizing CYPs are promiscuous re: substrate, but some CYPs are highly specific to substrate, e.g., Cyp 19 (aromatase w/ testosterone substrate). CYP enzyme levels induced by exposure to xenobiotics Broad substrate specficity = Low catalytic efficiency (Kcat). Drugs in general have t1/2 of 3 30 hrs, while endogenous substrates are much shorter (sec or min) reflects lower Kcat of drug CYPs. Broad substrate specificity of many CYPs accounts for much of reason for drug drug interactions. Multiple drugs may compete for binding site, altering metabolism of one or more of the drugs, thereby increasing t1/2 and plasma levels (Dixogin example). 5

6 Cytochrome P450 s Limted number of CYPs (15) in families 1 3 are mostly responsible for drug metabolism. In humans, 12 CYPs in particular important: CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5. The most active in drug metabolism are the 2C, 2D, and 3A subfamiles, with CYP3A4 involved in metabolism of ~50% of clinically used drugs. Cytochrome P450 Substrate Binding P450s possess active HEME group: 2 types of substrate binding to P450 Type 1 Binding: The substrate binds to a protein moiety near the active site. This brings the substrate within the region of the catalytic activity of the Heme-Fe. Fe in Low Spin Type II Binding: The substrate binds directly to the Heme-Fe. Fe in High Spin Can determine whether Type I or Type II binding spectrophotometrically -Type I binding of Cyt. P450 to the substrate gives a characteristic A MAX of around 390nm. -Type II binding of Cyt. P450 to the substrate gives a characteristic A MAX of around 420nm. 6

7 Systems Involved in Phase II Metabolism Four primary enzymes: 1. Glucuronosyltransferase glucuronic acid 2. Sulfotransferase sulfate 3. Glutathione-S-transferase glutathione (GSH) 4. Acetyltransferase acetyl enzymes are, for the most part, in the cytosol of cells In all cases (except GSH) requires conjugate activated to an Electrophilic Activated Donor Phase II Enzymes: Examples Glucuronidation and Sulfation of a Hydroxyl Group 7

8 UDP Conjugation - Phenytoin Phenytoin Common antiepileptic Marketed as Phenytek, Dilantin Insoluble in water Stabilizes inactive state of voltage-gated Na channels Induces CYP3A4, 2C19 Interactions: Coumarin and Warfarin increase serum phenytoin levels and prolong the serum half-life of phenytoin by inhibiting its metabolism Pharmacokinetics can be saturable, leading to highly variable plasma levels across minor dose changes Bioactivation and UDP Conjugation: Irinotecan Irinotecan cancer drug (mostly colon cancer). Topoisomerase 1 inhibitor leads to inhibition of DNA replication Administered as ProDrug Carboxyesterase activates to active drug (SN-38) 8

9 Gilbert s Syndrome Drug Polymorphism interaction Generally benign condition present in ~10% of pop n. Diagnosed clinically because circulating bilirubin levels are 60 70% elevated. Most common genetic polymorph associated with Gilbert s is mutation in UGT1A1 gene promoter, leading to reduced expression of UGT1A1. If drug undergoes Phase II with UGT1A1, it will compete with bilirubin metabolism conjugation, leading to reduced bilirubin elimination and build up in plasma. Glutathione-S-Transferase (GST) Thiol strong nucleophile, also susceptible to electrophilic attack +ve S H GST - Conjugates glutathione (GSH) onto substrates Tripeptide (cysteine/glutamine amide bond through glutamate R group). Present in all cells high in hepatocytes ~ 5mM 9

10 Factors that Affect Drug Biotransformation Species, strain, and genetic variation Age Diet Exposure to other drugs / xenobiotics Species Differences in the Duration of Action and Metabolism of Hexobarbital Factors that Affect Xenobiotic Biotransformation Substantial genetic (polymorphisms) exist in humans that impact drug/xenobiotic metabolism Clin Transplant Aug-Sep;23(4): Epub 2009 Apr 17. Influence of genetic polymorphisms in GSTM1, GSTM3, GSTT1 and GSTP1 on allograft outcome in renal transplant recipients. Singh R, Manchanda PK, Kesarwani P, Srivastava A, Mittal RD. Department of Urology and Renal Transplantation, Sanjay Gandhi Post Graduate Institute of Medical Sciences, Uttar Pradesh, India. Breast Cancer Res Treat Dec 25. [Epub ahead of print] Four polymorphisms in cytochrome P450 1A1 (CYP1A1) gene and breast cancer risk: a meta-analysis. Sergentanis TN, Economopoulos KP. School of Medicine, National University of Athens, Athens, Greece, tsergentanis@sni.gr. J Hum Genet Oct;54(10): Epub 2009 Aug 21. Association between polymorphisms in glutathione S-transferase Mu3 and IgG titer levels in serum against Helicobacter pylori. Tatemichi M, Iwasaki M, Sasazuki S, Tsugane S. Department of Hygiene and Preventive Medicine, School of Medicine, Showa University, Shinagawa-ku, Tokyo, Japan. tatemichi@med.showa-u.ac.jp 10

11 Bimodal Distribution of Patients: Those who Rapidly Inactivate Isoniazid and Those who Slowly Metabolize It Isoniazid First-line antituberculosis medication in prevention and treatment Metabolized in the liver via acetylation. Two forms of the enzyme responsible for acetylation - Some patients metabolize the drug more quickly than others 11

12 Age as Affecting Xenobiotic Biotransformation E.g., jaundice in newborns due to poorly induced glucuronidation of heme bilirubin build up Drug Metabolic Activity Schematic representation of the ontogeny of hepatic drug metabolic activity Environmental Factors Affecting Biotransformaiton Induction of Xenobiotic Metabolizing Systems 1. Many chemicals can induce the synthesis of the enzymes involved in Phase I and II xenobiotic metabolism and include chemicals found in the environment, the diet, and other drugs. 2. Inducers often exhibit specificity for the enzymes which they induce 12

13 Induction of Xenobiotic Metabolizing Systems Aryl Hydrocarbon Receptor System Expert Opin. Drug Metab. Toxicol. (2005) 1(1) CYP450 expression Nuclear Receptor Pregnane X Receptor (PXR) Transcriptional regulator of CYP3A4 Binds to response element of CYP3A4 promoter as a heterodimer with the 9-cis retinoic acid receptor (RXR). 13

14 Bioactivation and Toxicity Chemical Nature of Reactive Intermediates: Electrophiles Form covalent (irreversible) bonds with cellular nucleophiles such as GSH, proteins and DNA Free Radicals Odd or unpaired electron Can act as electrophiles Can abstract hydrogen from target molecules, such as lipids or nucleic acids Can activate molecular oxygen Acetaminophen toxicity O HN C CH 3 OH acetaminophen tablets = overdose Severe liver centrilobular necrosis -also renal effects Can be fatal common means of suicide 14

15 Metabolism ~40% Glucuronidatoin Minor (CYP2E) ~60% Sulfation NABQI N-acetyl benzoquinone imine TOXIC Electrophile -forms adducts with proteins, Bioactivation of Acetaminophen Relationship between hepatic glutathione levels and covalent binding of acetaminophen to target nucleophiles (proteins) Toxicity to acetaminophen occurs when GSH is depleted by > 80% 15

16 Potentially Toxic Drugs / Intermediates are: Electrophiles Modeling and Prediction Software has helped identify ROS - generating candidate lead structures potentially toxic in parent for or likely to yield reactive intermediates Febrifugine Derived from the Chinese herb Chang Shan Used traditionally in the treatment of malaria Use associated with hepatotoxicity 16

17 Overview of Pharmacokinetics Rates of Absorption, Disposition, Metabolism, Elimination Drive Therapeutic Dosing 12 Plasma Concentration TOXIC RANGE THERAPEUTIC RANGE SUB-THERAPEUTIC Dose 17

18 Influence of Variations in Relative Rates of Absorption and Elimination on Plasma Concentration of an Orally Administered Drug Plasma concentration Ka/Ke=10 Ka/Ke=1 Ka/Ke=0.1 Ka/Ke= TIME (hours) LOCUS OF ACTION RECEPTORS TISSUE RESERVOIRS Bound Free Free Bound ABSORPTION Free Drug EXCRETION Bound Drug SYSTEMIC CIRCULATION BIOTRANSFORMATION 18

19 Elimination Zero order: constant rate of elimination irrespective of plasma concentration. Rate of elimination = Amount constant First order: rate of elimination proportional to plasma concentration. Constant Fraction of drug eliminated per unit time. Rate of elimination = K x Amount 19

20 Plasma Concentration Profile after a Single I.V. Injection Plasma Concentration Distribution and Elimination C 0 Elimination only Distribution equilibrium Time lnc t = lnc o K el.t V d = Dose/C 0 t 1/2 = 0.693/K el When t = 0, C = C 0, i.e., the concentration at time zero when distribution is complete and elimination has not started yet. Use this value and the dose to calculate V d. 20

21 Pharmacokinetic parameters Get equation of regression line; from it get K el, C 0, and AUC Volume of distribution V d = DOSE / C 0 Plasma clearance Cl = K el.v d plasma half-life t1/2 = / K el Bioavailability (AUC) x / (AUC) iv 21

22 The half-life of elimination of a drug (and its residence in the body) depends on its clearance and its volume of distribution Rate of elimination = K el x Amount in body Rate of elimination = CL x Plasma Concentration (CL in units vol/time) Therefore, K el x Amount = CL x Concentration K el = CL/V d 0.693/t1/2 = CL/V d t1/2 = x V d /CL 22