Chimica Farmaceutica. Pharmacokinetics and related topics

Transcription

1 Chimica Farmaceutica Pharmacokinetics and related topics

2 INTRODUCTION In order to produce its intended effect, a drug must be present at an appropriate concentration in the fluid surrounding the effect site, that is, the biophase. Only rarely can drugs be applied directly to the biophase; in most cases drugs need to be transferred from the site of administration to the biophase. Usually, this translocation involves two steps: absorption and distribution. During absorption, the drug passes from its site of administration into the systemic circulation. Subsequently, the drug is distributed via the circulating blood plasma (the fluid portion of the blood) to the different parts of the organism, including the organ(s) in which the biophase for the drug is localized. Each drug molecule that reaches the target site can add to the intended pharmacological effect of the drug. However, at all times a portion of the drug molecules in the body is also distributed to organs and tissues that account for an irreversible loss of drug molecules from the body (drug elimination) by either biotransformation (the conversion of one chemical entity to another) or excretion. This causes a decrease in the concentration of the drug in the body and, consequently, also in the biophase. Corso di Laurea in Biotecnologie Pagina 2

3 INTRODUCTION A schematic representation of the processes involved in the journey of a drug molecule through the human body is shown in the left Figure. In the right Figure is shows a more detailed scheme of the main routes of drug absorption, distribution and elimination. Pharmacokinetics is the study of the drug concentrations in the different parts of the organism as a function of time. These concentrations depend on the dose administered and upon the rate and extent of absorption, distribution and elimination. Corso di Laurea in Biotecnologie Pagina 3

4 Pharmacodynamics and Pharmacokinetics Previously, we looked at the molecular targets for drugs and the mechanisms by which drugs interact with these targets. Later, we shall look at how drugs can be designed to optimize binding interactions with their targets. This is an area of medicinal chemistry known as pharmacodynamics. However, the compound with the best binding interactions for a target is not necessarily the best drug to use in medicine. This is because a clinically useful drug has to travel through the body in order to reach its target. There are many barriers and hurdles in its way and, as far as the drug is concerned, it is a long and arduous journey. The study of how a drug reaches it target, and what happens to it during that journey, is known as pharmacokinetics. When carrying out a drug design programme, it is important to study pharmacokinetics alongside pharmacodynamics. There is no point perfecting a compound with superb drug-target interactions if it has no chance of reaching its target. The four main topics to consider in pharmacokinetics are absorption, distribution, metabolism and excretion (often abbreviated to ADME) (and toxicology) ADMET Corso di Laurea in Biotecnologie Pagina 4

5 PASSAGE OF DRUGS THROUGH BIOLOGICAL BARRIERS On its journey through the body, a drug needs to cross different biological barriers. These barriers can be a single layer of cells (e.g. the intestinal epithelium), several layers of cells (e.g. in the skin), or the cell membrane itself (e.g. to reach an intracellular receptor). A drug can cross a cell layer either by traveling through the cells (transcellular drug transport) or through gaps between the cells (paracellular drug transport). Transcellular drug transport In order to travel through a cell or to reach a target inside a cell, a drug molecule must be able to traverse the cell membrane. The cell membrane (also called plasma membrane) is a lipid bilayer interspersed with carbohydrates and proteins. Although cell membranes largely vary in their permeability characteristics depending on the tissue, the main mechanisms of drugs passing through the cell membrane are passive diffusion, carrier-mediated processes and vesicular transport. Corso di Laurea in Biotecnologie Pagina 5

6 Passive diffusion Passive diffusion is the process by which molecules spontaneously diffuse from a region of higher concentration (e.g. outside of the cell) to a region of lower concentra tion (e.g. inside the cell), and it is the main mechanism for passage of drugs through membranes. Lipid-soluble drugs penetrate the lipid cell membrane with ease, and can pass the cell membrane by passive diffusion. Polar molecules and ionized compounds, on the other hand, partition poorly into lipids and are not able to diffuse through the cell membrane or do so at a much lower rate. Also, large molecules, such as proteins and protein-bound drugs, cannot diffuse through the cell membrane. Transmembrane diffusion is driven by the concentration gradient of the drug over the cell membrane. The rate of diffusion depends, apart from the lipid/water partition coefficient of the drug (P) and the concentration gradient (C_out - C_in), on membrane properties such as the membrane area (A) and thickness (h), and the diffusion coefficient (D) of the drug in the membrane, according to Fick's law: Corso di Laurea in Biotecnologie Pagina 6

7 Passive diffusion Many drugs are acidic or basic compounds, which are ionized to a certain degree in aqueous medium. Their degree of ionization depends on their dissociation constant (pka) and the ph of the solution, according to the Henderson-Hasselbach equation: Very weak acids with pka values higher than 7.5, are essentially unionized at physiological ph values. For these drugs diffusion over the cell membrane is rapid and independent of ph changes within the body, provided the unionized form of the drug is lipid soluble. For acidic drugs with a pka value between 3.0 and 7.5, the fraction of unionized drug varies with the changes in ph encountered in the organism. For these drugs the ph of the extracellular environment is critical in determining the diffusion across the cell membrane. For acidic drugs with a pka lower than 2.5, the fraction of unionized drug is low at any physiological ph, resulting in very slow diffusion across membranes. A similar analysis can be made for bases. At the diffusion equilibrium, the concentrations of unionized molecules on both sides of a biological barrier are equal. If the ph on both sides of the barrier is equal, then the concentration of ionized molecules and, consequently, the total concentration of the molecules, will be the same on both sides of the barrier. However, if there is a difference in ph, as e.g. between blood plasma (ph 7.4) and stomach contents (ph 1-3), the concentration of the ionized molecules at equilibrium, and, therefore, the total concentration, will be much higher on one side of the barrier than on the other. This phenomenon is called ion trapping. Corso di Laurea in Biotecnologie Pagina 7

8 Carrier-mediated processes Many cell membranes possess specialized transport mechanisms that regulate entry and exit of physiologically important molecules and drugs. Such transport systems involve a carrier molecule, that is, a transmembrane protein that binds one or more molecules and releases them on the other side of the membrane. Such systems may operate passively (without any energy source) and along a concentration gradient; this is called "facilitated diffusion." However, facilitated diffusion seems to play only a minor role in drug transport. An example is the transport of vitamin B12 across the GI membrane. Alternatively, the system may spend energy (obtained from the energy rich molecule adenosine triphosphate (ATP) required to pump molecules against a concentration gradient; this mechanism is called "active transport." At high drug concentrations the carrier sites become saturated, and the rate of transport does not further increase with concentration. Furthermore, competitive inhibition of transport can occur if another substrate for this carrier is present. In recent years, several transporters have been described to be present in various organs and tissues throughout the body and to determine absorption, distribution and elimination of compounds that are substrates for these transporters. Although some transporters mediate the uptake of compounds in the cell (influx transporters), others may mediate secretion back out of the cell (efflux transporters). Transporters in the intestinal membrane affect the absorption of drugs, while transporters in the liver and kidney influence elimination by mediating transport into and out of cells responsible for biotransformation (hepatocytes) or excretion (e.g. renal tubule cells in the kidneys). Furthermore, efflux transporters may limit the penetration of compounds into certain areas of the body, such as the cerebrospinal fluid and blood cells. Corso di Laurea in Biotecnologie Pagina 8

9 Vesicular transport During vesicular transport the cell membrane forms a small cavity that gradually surrounds particles or macromolecules, thereby internalizing them into the cell in the form of a vesicle or vacuole. Vesicular transport is the proposed process for the absorption of orally administered Sabin polio vaccine and of various large proteins. It is called endocytosis when moving a macromolecule into a cell, exocytosis when moving a macromolecule out of a cell, and transcytosis when moving a macromolecule across a cell. Paracellular drug transport Drugs can also cross a cell layer through the small aqueous contact points (cell junctions) between cells. This paracellular drug transport can be initiated by a concentration gradient over the cell layer (passive diffusion), or by a hydrostatic pressure gradient across the cell layer (filtration). The size and characteristics of cell junctions widely vary between different barriers to drug transport. For example, the endothelium of glomerular capillaries in the kidney forms a leaky barrier, which is very rich in intercellular pores. Therefore, this membrane is very permeable and permits filtration of water and solutes. On the other hand, endothelial cells of brain capillaries are sealed together by tight junctions, practically eliminating the possibility of paracellular drug transport. Corso di Laurea in Biotecnologie Pagina 9

10 Drug absorption Absorption can be defined as the passage of a drug from its site of administration into the systemic circulation. If a drug is administered directly into the systemic circulation by intravenous (i.v.) administration, absorption is not needed. Drugs can be administered by enteral and parenteral routes. Enteral administration occurs through the GI tract, by contact of the drug with the mucosa in the mouth (buccal or sublingual), by swallowing (oral) or by rectal administration. In parenteral administration the GI tract is bypassed; examples are the i.v. (direct injection in the systemic circulation) and intramuscular (injection into a muscle) routes. Drugs can also be absorbed through the skin or through the mucosa of various organs (e.g. bronchi, nose, and vagina). In some cases, a drug is applied for a local effect, and no absorption is intended (e.g. antacids that neutralize stomach acid). In this chapter, we will describe drug administration by the oral route, which is the most common and popular route of drug dosing. Corso di Laurea in Biotecnologie Pagina 10

11 Common Routes of Drug Administration Corso di Laurea in Biotecnologie Pagina 11

12 Common Routes of Drug Administration Corso di Laurea in Biotecnologie Pagina 12

13 Common Routes of Drug Administration Corso di Laurea in Biotecnologie Pagina 13

14 Drug absorption Drug absorption refers to the route or method by which a drug reaches the blood supply. This in turn depends on how the drug is administered. The most common and preferred method of administering drugs is the oral route and so we shall first concentrate on the various barriers and problems associated with oral delivery. An orally taken drug enters the gastrointestinal tract (GIT), which comprises the mouth, throat, stomach, and the upper and lower intestines. A certain amount of the drug may be absorbed through the mucosal membranes of the mouth, but most passes down into the stomach where it encounters gastric juices and hydrochloric acid. These chemicals aid in the digestion of food and will treat drugs in a similar fashion if the drug is susceptible to breakdown. For example, the first penicillin used clinically was broken down in the stomach and had to be administered by injection. Other acid-labile drugs, such as local anaesthetics or insulin, cannot be given orally. Corso di Laurea in Biotecnologie Pagina 14

15 Drug absorption If the drug does survive the stomach, it enters the upper intestine where it encounters digestive enzymes that serve to break down food. Assuming the drug survives this attack, it then has to pass through the cells lining the intestinal or gut wall. This means that the drug has to pass through a cell membrane on two occasions, first to enter the cell and then to exit it on the other side. Once the drug has passed through the cells of the gut wall, it can enter the blood supply relatively easily as the cells lining the blood vessels have pores between them through which most drugs can pass. In other words, drugs enter the blood vessels by passing between cells rather than through them. The drug is now transported in the blood to the body's 'customs office': the liver. The liver contains enzymes which are ready and waiting to intercept foreign chemicals, and modify them such that they are more easily excreted: a process called drug metabolism. Corso di Laurea in Biotecnologie Pagina 15

16 It can be seen that stringent demands are made on any orally taken drug. The drug must be chemically stable to survive the stomach acids, and metabolically stable to survive the digestive enzymes in the GIT as well as the metabolic enzymes in the liver. It must also have the correct balance of water versus fat solubility. If the drug is too polar (hydrophilic), it will fail to pass through the fatty cell membranes of the gut wall. On the other hand, if the drug is too fatty (hydrophobic), it will be poorly soluble in the gut and will dissolve in fat globules. This means that there will be poor surface contact with the gut wall, resulting in poor absorption. Drug absorption Corso di Laurea in Biotecnologie Pagina 16

17 Drug absorption It is noticeable how many drugs contain an amine functional group. There are good reasons for this. Amines are often involved in a drug's binding interactions with its target. However, they are also an answer to the problem of balancing the dual requirements of water and fat solubility. Amines are weak bases, and it is found that many of the most effective drugs are amines having a pka value in the range 6-8. In other words, they are partially ionized at blood ph (~7.4) and can easily equilibrate between their ionized and non-ionized forms. This allows them to cross cell membranes in the non-ionized form, while the presence of the ionized form gives the drug good water solubility and permits good binding interactions with its target binding site. Corso di Laurea in Biotecnologie Pagina 17

18 Henderson-Hasselbalch equation The extent of ionization at a particular ph can be determined by the Henderson- Hasselbalch equation: where [RNH2] is the concentration of the free base and [RNH3+] is the concentration of the ionized amine. Ka is the equilibrium constant for the equilibrium shown, and the Henderson-Hasselbalch equation can be derived from the equilibrium constant. Note that when the concentration of the ionized and unionized amines are identical (i.e. when [RNH2 = [RNH3+]), the ratio ([RNH2/[RNH3+]) is 1. Since log 1 = 0, the Henderson-Hasselbalch equation will simplify to ph = pka. In other words, when the amine is 50% ionized, ph = pka. Therefore, drugs with a pka of 6-8 are approximately 50% ionized at blood ph (7.4). Corso di Laurea in Biotecnologie Pagina 18

19 Key Points on Drug Absorption Pharmacodynamics is the study of how drugs interact with a molecular target, whereas pharmacokinetics is the study of how a drug reaches its target in the body and how it is affected on that journey. The four main issues in pharmacokinetics are absorption, distribution, metabolism, and excretion. Orally taken drugs have to be chemically stable to survive the acidic conditions of the stomach, and metabolically stable to survive digestive and metabolic enzymes. Orally taken drugs must be sufficiently polar to dissolve in the git and blood supply, but sufficiently fatty to pass through cell membranes. Most orally taken drugs obey Lipinski's rule of five and have no more than seven rotatable bonds. Highly polar drugs can be orally active if they are small enough to pass between the cells of the gut wall, are recognized by carrier proteins, or are taken across the gut wall by pinocytosis. Distribution round the blood supply is rapid. Distribution to the interstitial fluid surrounding tissues and organs is rapid if the drug Is not bound to plasma proteins. Some drugs have to enter cells in order to reach their target. A certain oercentage of a drug may be absorbed into fatty tissue and/or bound to macromolecules. Drugs entering the CNS have to cross the blood-brain barrier. Polar drugs are unable to cross this barrier unless they make use of carrier proteins or are taken across by pinocytosis. Some drugs cross the placental barrier into the fetus and may harm development or prove toxic in newborn babies Corso di Laurea in Biotecnologie Pagina 19