ABSTRACT
This study was undertaken to compare the effect of vitamin C on the pharmacokinetics of chloroquine using human volunteers. Quality control studies for both chloroquine and vitamin C; identification test, assay, weight variation, friability, disintegration time and dissolution rate were carried out according to BP 2009. Jayanathi et al., 1994 method was adopted and validated based on ICH guideline for this study. Calibration curve was constructed within the range of 2 – 10 µg/ml and validated. The adopted method was then employed in determining the effect of vitamin C on the pharmacokinetics parameters (Cmax, Tmax, Ka, Ke, t1/2a, t1/2e, Cl, Vd, lag time and AUC) of chloroquine using human volunteers of 30 years and above and the study was divided into three phases with a washout period of three weeks. Both chloroquine and vitamin C used for this study were found to have the labeled active ingredient. They all passed the assay test as they were within the acceptable limits 95 – 105 %. Weight variation was conducted for both chloroquine and vitamin C and they all passed as their percentage mean deviation was less than 5 %. Similarly, both drugs passed the friability test which was less than 1%. Disintegration tests for both drugs reveals that they disintegrate in less 5 mins and also both chloroquine and vitamin C passed the dissolution rate as more than 70 % of the active ingredient was released in 30 mins. The percentage recovery of the method was within the accepted range of 98 – 102 %. Calibration curve was linear within the range of 2 – 10 µg/ml as the correlation coefficient was 0.9905. The regression equation was y= 0.001x + 0.120. When chloroquine was administered alone no significant difference (p<0.05) at lower values was observed, however, on co-administration of chloroquine and vitamin C no significant difference (p<0.05) at lower values was also observed, except for clearance (Cl) which was statistically significant (p<0.05) at lower values. On the other hand, when an hour delayed administration of chloroquine and vitamin C no significant difference (p<0.05) was observed, except for clearance (Cl) which was also statistically significant (p<0.05) at lower values. This finding indicates that vitamin C may influence the pharmacokinetics of chloroquine when co-administered together depending on the time of administration of the two drugs.
CHAPTER ONE
1.0 INTRODUCTION
1.1 Drug interaction
It is evident that in some cases when two or more drugs are administered concurrently or at an interval of time, the expected pharmacological action is not being obtained. Sometimes potentiation of action of one of the drug may be observed. While on the other hand a diminished in action of the other drug occurs in some cases. This could be best explained through the study of possible interaction between the drugs in question. This is termed as drug – drug interaction study. (Pixtrim et al., 1995)
There are many ways in which drug administrated by any route may interact in the patient to produce a harmful effect. Some of these interactions occur only when two drugs are administered within hours, days or weeks of each other; and sometimes the interaction take place only after one of the drugs has been taken for several weeks. There are biochemical explanations for some of this interaction, but for others there is yet no information available about the basic mechanism involved.
1.1.1 General mechanism of action
The mechanism of drug interaction development can be best explained into 2 ways.
1.1.1.1 Pharmacokinetics interactions
A). Absorption
b). Distribution
c). Elimination
1.1.1.2 Pharmacodynamics interactions
- Enhanced effect produced by 2 drugs acting at the same site
- The increase effect produced by 2 drugs ‘at different receptors site.
- Enhanced effect of 2 drugs by one which is devoid of action itself.
- Antagonism of the effect of one drug by another.
Pharmacokinetics
Most of drugs interactions are kinetics in origin, one of the most useful pharmacokinetics concepts to have emerged in recent years particular in understanding interaction is that of the area inscribed by the plasma concentration. Time curve of a drug. After a single dose, this area (AUC) is a function of the dose, the fraction of the dose entering the general circulation and the drug clearance (Lappin et al., 2006)
This can be expressed as follows;
| AUC = D x F/C————————————————————————— | (1.1) |
Where;
AUC = area under curve
D = dose
C = drug clearance
F = fraction of the dose entering the general circulation
When a drug is given repetitively, accumulation occurs and the plasma concentration increase after each dose until a steady state is reached provided that:
- Absorption is not altered.
- Drug binding to plasma proteins remains constant over a wide concentration range.
- Elimination rate is not dose dependent.
- The drug does not induced its metabolism, then
| AUC (SD) = AUC (MD) ————————————————————- | (1.2) |
AUC (SD) = Area under curve during' single dose.
AUC (MD) = Area under curve during multiple dose.
1.2 Drugs Absorption Influence
Many drugs influence gastrointestinal function; the usual result of absorption interaction is a reduction in the rate of absorption or in the total amount of drug absorbed, so that drug effect are reduced or abolished. Interactions that cause therapeutic failure are obviously important but are unlikely to be recognized unless significantly looked for, on the other hand the risk of drug toxicity may be increased if absorption is enhanced. From the practical point of view, it is important to differentiate between interaction that alter the rate of drug absorption and those that increase or decrease the total amount of drug absorbed since the consequences may be quite different. A change in the rate of absorption of a long
- acting drug such as warfarin would probably have little effect or no effect, whereas a change in total amount absorbed may be disastrous. In contrast, if the absorption of a drug with a short biological half life such as procainamide is slowed down, therapeutic plasma concentration may never be reached. The absorption of drug from gastrointestinal tract is a complex process that depends on many physiological and physicochemical factors. This leads to many pathways within which the mechanisms of drugs absorption interaction are proposed (Skipper et al., 1996)
1.3 Mechanism of Drug Absorption Interaction
There are many proposed mechanism for drug absorption interaction. The possible ones are as follows:
1.3.1 PH effect on dissolution and ionization
Drug induced change in pH of gastrointestinal fluids may have complex and unpredictable effect on the absorption of other drugs taken at the same time.
According to the pH partition theory, weak organic acids are largely absorbed from the stomach, while weak bases are absorbed best from the more alkaline contents of the upper small intestine it is sometimes stated that the absorption of weak acid is reduced if they are given with alkaline drug, since less drug would be present in the unionized lipid soluble diffusible state (Amidon et al., 1995)
1.3.2 Change in gastric emptying and Gastro intestinal motility.
The stomach is not an important site of drug absorption. Basic drugs and compounds absorbed by active transport are not absorbed from the stomach to any extent, and even weakly acid drugs such as aspirin, warfarin, and barbiturates and low molecular weight natural compounds such as ethanol are absorbed much more slowly from the stomach than from the small intestine. Drugs are probably absorbed more rapidly from the upper small intestine than from the stomach, because of the much greater surface area of the intestine. The rate of gastric emptying may therefore limit to the rate of drug absorption and particularly important in the context of interactions since it can be influenced by many drugs. Drug effects can be reduced dramatically or even abolished if gastric emptying is retarded by food. In contrast, absorption is more rapid and toxicity can be greatly increased when drugs are given orally in the same dose in dilute form rather than concentrated solutions, and this effect has been attributed in part to rapid gastric emptying. (Amidon et al., 1995)
1.3.3 Formation of complexes, ion – pairs and chelates
Drugs may interact in the gastrointestinal tract to form complexes, ion pairs and chelates which may be absorbed more rapidly or more slowly than the parent drugs. The absorption of tetracycline is inhibited by the formation of insoluble chelates with metals such as Calcium and iron. Dicoumarol absorption is increased by the formation of more soluble chelates with magnesium hydroxide, and the absorption of quaternary ammonium anti arrhythmia agent is enhanced by ion-pair formation with salicylate and trichloroacetate. The absorption of drugs may also be reduced by absorption on to Kaolin or charcoal or binding to ionic exchange resins (Quintanar et al., 1997)
1.3.4 Interference with active transpor
Drugs that are analogues of naturally occurring purines, pyramidines, sugar and amino acid may be absorbed by small intestine active transport; and it has been suggested that absorption may be reduced by competition between substrates such as L-dopa and phenyl amino derived from dietary sources. It is possible for one drug to inhibit enzymes involved in the active transport of another drug and such an interaction has been postulated between chlorpromazine and L-dopa (Prescott, 1969).
1.3.5 Distribution of Lipid Micelles
Interferences with micelles formation may limit the solubility of lipids and inhibition of absorption of cholesterol, bile acids and Vitamin A by neomycin has been attributed to this mechanism (Shirley et al., 2000)
1.3.5 Change in portal blood flow
The splanchnic blood flow may occasionally be a rate limiting factor in drug absorption and this may be of clinical significance because many drugs could have direct effect on the local gastrointestinal blood flow (Prescott, 1969).
1.3.6 Toxic effect on gastrointestinal Mucosa
Toxic effect on gastrointestinal mucosa may cause mal-absorption syndrome with impaired absorption of other drugs. Neomycin, P-Amino salicylic acid and colchicines may cause megaloblastic anaemia through interference with vitamin B12 absorption (Amidon et al., 1995)
1.3.7 Change in Volume, composition and viscosity of secretion
Drugs can also influence the volume and composition of gastrointestinal secretions (including bile) and changes in viscosity may modify drug absorption. It is now recognized that many drugs interact through this mechanism. (Amidon et al., 1995
1.3.8 Effect on mucosal and bacterial drug metabolism
Recent findings have shown that many drugs are extensively metabolized by the gastrointestinal mucosa and the gut bacterial flora, and this process might be influenced by the concurrent administration of other drugs (Prescott, 1969).
1.3.9 Change in membrane permeability
During oral therapy, the intestinal mucosa is exposed intermittently to very high drug concentration that many alter the permeability of the gastrointestinal epitheliums. ln this context it is interesting to note that insulin and many polypeptides greatly enhanced the membrane transport of ranitidine, isoniazid, salicylate and chlorpromazine (drugs normally considered to cross cell membrane by passive diffusion). Thus, intestinal up take of ionized is enhanced by insulin and this effects is antagonized by ouabain. (Amidon et al., 1995)
1.4 Drugs Distribution Interaction
Competition between co-administered drugs at non specific binding sites in the body can result in displacement of one drug by another, with a resulting rise in the free and active fraction of a drug. lt is widely believed that this displacement or redistribution phenomenon causes the enhanced clinical effects and toxicity seen when certain drugs and other substances interact in man. The distribution of drugs is dependent on the following factors:- Regional blood, flow Partition coefficient of drug between blood and tissue, Binding plasma proteins and tissues macromolecules and Active transport, (Alstead et al., 1971) This mechanism of drug interactions can be broadly divided into those occurring at the receptor level or beyond and those occurring by pharmacokinetic process prior to the receptor. In the intact body, more than one mechanism may operate simultaneously. Many drugs reversibly bound to plasma proteins. Drugs may compete for common binding sites on the binding proteins and when administered together they may displace each other. As a result, there is tendency towards an increased in the free (unbound) plasma concentration. The potential importance of this interaction lies in the fact that only the unbound (free) fraction of drug in plasma is available for distribution to tissue (Alstead et al., 1971)
Although in theory this mechanism could produce potent drugs interaction. lt has been overemphasized in the past because unwanted extrapolations have been made from in-vivo studies. If a drug A displaced a second drug B from its plasma protein binding site, the increase in the unbound fraction of B will occur in-vivo will always be less than that observed in vitro. The reason for this is that a proportion of displaced drug molecules will diffuse extravascularly into the drugs distribution volume. Consequently the greater the distribution volume, the less will displacement interactions results in clinically important effect. In addition, displacement interaction will only be significant if the displaced drug is itself highly protein bound (more than 90 %) otherwise increase in free concentration will be too trivial to be relevant. The following considerations would help to determine whether a given interaction was purely re-distributional one involving plasma protein.
- The whole interaction should be mimicked by any drugs with comparable displacement ability.
- Absorptive interaction should be excluded e.g. the interaction should be shown to occur when both drugs are administered parentally or alternatively formed pharmacokinetic analysis of absorption studies.
Metabolic and excretory rates should be measured. Any changes should be entirely predictable from changes in the concentration of free drug. These conditions are not satisfied, some quantitative allowance must be made for the contributions of these and other pharmacokinetic models of interaction. (Alstead et al., 1971)
1.5 Drugs Elimination Interaction
The elimination rate of a drug can best be described by its clearance. Changes in renal function can modify a number of pharmacokinetic processes in the body and there by lead to unanticipated drug effects or drug interaction. Most of drugs are eliminated by renal excretion, by metabolism or by a combination of the two processes. Total clearance is equal to the sum of the individual clearance for renal excretion and metabolism.
1.5.1 Renal Excretion
Drug loss by the kidneys is determined by the net effect of Glomerular filtration, tubular secretion and tubular re-absorption, Glomerular filtration is not greatly influenced by other drugs although displacement for protein binding sites may lead to increase concentration of drug in Glomerular fluid and enhance renal elimination. Tubular secretion of acid drugs is mediated by relatively non – specific active transport system and a variety of endogenous and exogenous compounds are potential substrate. Competition for tubular secretion may lead to unpaired renal excretion of drugs whose major route of elimination is via this pathway. Thus probenecid impairs the elimination of penicillin salicylate and indomethacin. Tubular re-absorption is passive and dependent upon the lipid solubility with which a drug can cross the tubular epithelium and on the concentration gradient between tubular fluid and plasma water. Since many drugs are weak electrolytes, the pH of the tubular fluid will be a determinant of drug re-absorption. Consequently, the clearance of acid drug (e.g. salicylate) will be more rapidly excreted in acid urine. These changes in urine pH however will only be of practical importance for the elimination of those drugs for which renal excretion plays a significant role. (Alstead et al., 1971)
1.5.2 Drug Metabolism
Metabolism of drugs generally leads to the formation of polar (lipid – insoluble) derivatives which can undergo renal excretion. Many commonly used drugs are oxidized in the hepatic endoplasmic reticulum. The activity of the enzyme systems involved is subject to wide inter-individual variability. Moreover, the activity of this system may be enhanced (induced) or diminished (inhibited) by other drugs or environmental pollutants. Induction of hepatic microsomal oxidizing system leads to increased drug clearance and a reduced steady state level during multiple dose therapy. This may result in therapeutic failure. By contrast, inhibition of hepatic microsomal oxidation leads to reduced drug clearance and increased steady state plasma levels. Inhibition of microsomal oxidation is therefore liable to precipitate toxicity. The mechanisms involved in inhibition of drug metabolism are unclear. It is important, therefore to realize that a drug which inhibits the metabolism of one oxidized drug does not necessarily inhibit the metabolism of other oxidized drug. There are several factors that could influence metabolism, the most important ones are, genetic physiological factors, pharmacodynamics factors and environmental factors (Prescott, 1969).
1.6 Statement of Research Problem
Inappropriate poly-pharmacy has been reported by several studies to be an independent predictor of harmful drug-drug interaction and the consequent adverse drug reaction (Rodriguez and Oliveira, 2016; Nguyen et al., 2006). Ascorbic acid which is an antioxidant and a urinary acidifier, has been reported to affect the pharmacokinetic parameters of Pefloxacin which is also as weakly basic as Chloroquine (Awofisayo et al., 2012).Common co-prescription of vitamin C with Chloroquine (Ezenduka et al., 2014) may have arisen concern for possibility of pharmacokinetic interaction owing to the antioxidant and urinary acidifying properties of the former that may modify the absorption, metabolism and excretion of Chloroquine.
1.7 Justification
Poly-pharmacy in our healthcare settings may result in drug-drug interaction which may cause modification of pharmacokinetic parameters. Based on common co-prescription of vitamin C with chloroquine in our healthcare settings, there may be interaction which may result in the modification of the pharmacokinetic parameters of chloroquine. There is little information on whether the pharmacokinetics of chloroquine may be significantly modified by co-administration with Ascorbic acid. This study is designed to provide an insight on this identified gap.
1.8 Aim and Objectives of the Study
1.8.1 Aim
To determine the effect of vitamin C on the pharmacokinetic parameters of Chloroquine (CQ).
1.8.2 Objectives
The objectives of this study is to: Sample and carry out quality control assessment tests of both chloroquine and vitamin C tablets according to BP, 2009. adopt and validate analytical method for the determination of CQ in saliva. determine the pharmacokinetics of Chloroquine alone after concurrent administration with Vit C delayed administration with vitamin C in healthy human volunteers.
1.9 Research Hypothesis
Vitamin C has no significant effect on the pharmacokinetics of Chloroquine (CQ) when the two drugs are co-administered.
