Drug discovery: preclinical and clinical studies.

Preclinical studies: Molecules that have therapeutic potential must be evaluated thoroughly before they are used in clinical practice. Initially these drugs are evaluated in animals, which is called preclinical studies.

Objectives of animal trials:

·        Screening of various molecules for their therapeutic potential.

·        To find out efficacy.

·        To find out safety.

·        Investigating pharmacokinetic parameters.

·        Comparison with existing agents.

                              The basic aim behind all preclinical testing is judging the predictability of drugs in human beings.

Clinical trials: After establishment of safety and efficacy in animals, a drug can be tested in human beings. Any drug undergoes total 4 phases of clinical trials. After first 3 phases if the drug is found safe and effective then it is allowed in the market.

Objective of clinical trials:

·        Establishing safety.

·        Evaluating efficacy.

·        Studying pharmacokinetic parameter.

·        Finding mechanism of action.

·        Comparison with existing treatment.

ETHICS OF RESEARCH IN MAN

1.      The healthy human volunteer or patient volunteers should have the right to choose for themselves whether or not they will participate in research, i.e. they have the "right to autonomy". The investigating scientist or physician has no right to choose "martyrs for society through persuasion or lure".         

                              Hence the law requires that the physician should obtain the written inform consent from every human volunteers. The basic elements of such informed consent includes:

a.      a fair explanation of the procedures to be followed.

b.      an explanation of the possible discomfort and risks.

c.      a description of the anticipated benefits (particularly when the volunteers are like cancer patients).

d.      a permanent offer to answer any query related to the procedure.

e.      an assurance that the subject is free to withdraw his consent and to discontinue participation in the project at any time.

f.       an explanation of compensation or available treatment if adverse effects appears.

2.      Women with child bearing potential are never subjected to clinical trials unless the rigorous teratogenic studies in animals have been completed. Also, with an obvious exception of clinical trials with new contraceptive drugs, no women should be used as a subject if they are willing to bear the child either during or immediately after the end of clinical trials.

3.      Use of control drugs: Suppose a drug is found to improve some symptoms in some diseases, it may not be the effect of the drug. In can be a natural course of disease (e.g. symptoms of common cold is self-limiting after 2-3 days). To conform that the improvement is due to the test drug, it is compared with placebo, some patients in the trial will receive placebo (dummy drug) while some will receive test drug. If improvement is observed only in the group receiving test drug and if the difference is statistically significant the drug is said to be effective. Similarly test drug is compared with the standard drug that has similar type of action. Studies according to agent compared are called as placebo controlled (test drug X placebo) or positive controlled (test drug X standard treatment) studies or uncontrolled (no control).

4.      Randomization: It is the random allotment of test drug or placebo to the patients. Randomization is done with the help of table of random number or computerized programs. According to that fixed pattern patients in the trial will get either placebo or test drug. It avoids bias in the recruitment.

5.      Blinding:

                           I.          Single blind trial: if individual in the trial knows that he is receiving placebo or test drug, the psychological interpretations may alter the effect. To avoid this, patients are not informed whether they are receiving placebo or test drug. This is single blind trial. To avoid bias.

                         II.          Double blind trial: if the investigator in the trial knows what pateint has received, drug or placebo, his psychological factor can alter the results. In double blind trial neither patient nor investigator knows who is taking test medication and who is taking placebo. This design of trials avoids bias of patient and investigator.

6.      Cross over design of trial: Suppose in a clinical trial, individuals in group A receive test drug and tose in group B  receive placebo, after completion of the trial there is a drug free period (wash out period) to abolish the total effect of drug. Then individuals in group A will receive placebo and those in group B will get test drug. The analysis of two steps is done together. This cross over design of the trial reduces individual variation in the trial and needs small number of patients.

7.      Prospective and retro prospective trials: When objective of the trials are decided first and then the drug is tested it is prospective trial. In retro prospective trial, first the drug is tested, data is collected and analyzed without specific objectives in mind.

NEW DRUG

The main stage of drug development process namely

       I.          The discovery phase- i.e. the identification of new drug entity as a potential therapeutic agent

     II.          The development phase- during which the compound is tested for safety and efficacy in one or more clinical indication, and then suitable formulations and dosage forms devised.

The main aim is to achieve registration by one or more regulatory authorities to allow the drug to be marketed legally as a medicine for use.

Broadly the process can be divided into three components.

                 I.          Drug discovery: during which candidate molecules are choosen on the basis of their pharmacological properties.

               II.          Preclinical development: during which a wide range of non-human studies (e.g. toxicity testing, pharmacokinetic analysis and formulations) are performed.

              III.          Clinical development: during which the selected compound is tested for efficacy, side effects and potential danger in volunteers and patients.

Target selection: The stage of development of a typical new drug i.e, synthetic compound being developed for systemic use has target selection. That drug targets are functional proteins (receptors, enzymes, transport proteins, ion channels).

PRECLINICAL EVALUATION

Initially, animal studies are performed to define the pharmacological profile of the lead compound. The screening may be either organ oriented or disease oriented.

               The major areas of preclinical evaluation are:

1.      Pharmacodynamics studies: Here action relevant to the proposed therapeutic use (and other effects) are studied on animals.

For example, in search of antihypertensive activity of the lead compound, the study can be undertaken on dogs, cats or rats to find out other cardiac effects like ECG changes and ionotropic-chronotropic effects, cardiac output and total peripheral resistance. Once the lead compound exhibits promising results, the studies can be further made at cellular level. Depending on the results, the studies can be further extended to molecular level to find out receptor affinity and selectivity. The graded response assay or quantal assays are then performed to find out ED₅₀ of the drug.

2.      Toxicological studies: If the agent possessed useful activity, it should now be studied for possible adverse effect on major organ system. The major kinds of information needed from preclinical studies are:

a.      Acute toxicity: The aim is to find out acute dose that is lethal to 50% of animals (LD₅₀). The study is done at least on two species of animals and the drug is given in graded doses to several groups of animals by at least two routes, one of which should be the proposed route to used in human beings.

b.      Sub-acute toxicity: The aim is to identify the target organs susceptible to drug toxoicity. Three doses are used in two animal species. The animals are maintained at maximum tolerated doses for a minimum period of four weeks to a maximum of three months, so as to allow development of pathological changes. Therefore, biochemical and haematological changes are evaluated. Finally, the animals are killed and subjected to histopathological examination.

c.      Chronic toxicity: The goals are same as that of the sub-acute toxicity. However such studies are especially important if the drug is intended for chronic use in human beings. Usually two animal species (one rodent and one non-rodent) are used. The duration of study may range from one to two years.

d.      Special toxicity: Toxicological data on teratogenicity, mutagenicity and carcinogenicity is done. As has become mandatory after the unfortunate episode of thalidomide disaster in 1950 which has left more than 10,000 new born congenitally deformed and crippled due to phocomelia.

3.      Pharmacokinetic studies: After performing toxicological studies, the promising test compound is subjected to pharmacokinetic studies in several species of animals like rats, dogs and sometimes monkeys. Besides studying its absorption, distribution, metabolism and elimination, these studies also establishes their relative bioavailability after its oral and parenteral administration. Its elimination half-life (t_1/2) is also estimated through pharmacokinetic data.

4.      Assessment of safety index: From the toxicological and pharmacokinetic data, the LD₅₀ and ED₅₀ of test compound are found, respectively. From these data their therapeutic index and certain safety factors are calculated.

CLINICAL TRIALS (HUMAN STUDIES)

When a compound deserving trial in man is identified by animal studies, the regulatory authorities are approached, who on satisfaction, issues an ‘Investigational new drug (IND) licence’. The drug is formulated into suitable dosage form and clinical trials are conducted in a logical phased manner. To minimize any risk, initially few subjects receive the drug under close supervision. Later, larger members are treated with only relevant monitoring. Standards for the design, ethics, conduct, monitoring standards recording and analyzing data and reporting of clinical trials have been laid down into the form of ‘Good Clinical Practice (GCP)’ guidelines by International Conference of Harmonization (ICH). Adherence to these provides assurance that the data and reported results are credible and accurate, and that the rights, integrity and confidentiality of trial subjects are protected. The clinical studies are conventionally divided into 4 phases.

Phase I (Human pharmacology and safety): It is a phase of clinical pharmacological evaluation of the new drug and is performed on small number (20-25) of healthy volunteers. If the drug is expected to have significant toxicity (as in the case of anti-cancer drugs or drugs to be used in AIDS therapy), the volunteers with particular disease are rather than healthy volunteers. The objective of this necessary, but caution phase of investigation are

       i.          To determine whether humans or animals show significant pharmacokinetic difference.

      ii.          To determine a safe and tolerated dose, in human. The selection of initial human dose is difficult because the toxicological data on animals are limited usefulness (quantitatively) for selecting such a dose – the common rule is to begin 1/5^th to 1/10^th of the maximum tolerated dose (mg/kg) in animals and calculating it for and average  human body weight of 70 kg. the drug is then given in small investment till the therapeutically effective dose is attained by clinical observation.

     iii.          To determine pharmacokinetic of the drug in humans so as to decide whether the deficiency in drug effect, if any, is as a result of its lack of absorption or its faster elimination.

     iv.          To detect any predictable toxicity.

These trials are NON-BLIND or OPEN LABEL; that means both the investigator and subject knows what is being given. Phase I trials are usually performed by clinical pharmacologists in a research centre especially equipped for pharmacokinetic studies.

PHASE II (Therapeutic exploration and dose ranging): In this phase the drug is studied for the first time in the patients with target disease, to determine its efficacy. These trials are divided into EARLY and LATE PHASES.

               In the EARLY PHASE, a small number of patients (20-200) are studied in detail to observe the potential therapeutic benefits and side effects. This idea is to establish a dose range for a more definitive therapeutic trial to be undertaken in the LATE PHASE. It is usually a SINGLE BLIND design where only the subject does not know whether he is taking an inert placebo (if used) or a positive control (an established standard medicine) or the new drug (under trial).

               The LATE PHASE trials are conducted on a large number of patients (50-300) in a controlled DOUBLE BLIND manner, where the investigator is also ignored (besides the subject) whether he is prescribing a placebo, or a positive control medicine, or the new drug under trial. This is done to rule out the influence of preconvinced notion or benign communication by the investigator to his subject. In such a design, a third party holds the code identifying each medication and this code is not dispatched until all the clinical data have been collected.

               In short, phase II trials are a carefully controlled blind study (single as well as double) to ensure safety and efficacy of the new drug in a specific disease and to compare these data with that of the standard drug used for the same disease.

Phase III (Therapeutic conformation/ comparison): These are large scale randomized control trials in patients (25-1000 plus) to further establish the safety and efficacy. These are designed to minimise errors in the information gathered in phase I and phase II trials. Therefore these trials are made using DOUBLE BLIND CROSS OVER design; that means the standard drug, the placebo and the new drug are given in alternating periods and the sequence is systematically varied so that different subsets of patients receive each of the possible sequence of the treatment. The phase III trials are conducted by a large number of clinicians at different centres. It may take an average of five years to be completed.

New drug application: Once phase III is completed satisfactorily the sponsors can file a “New Drug Application” with the drug control authorities of that country. The new drug application usually contains thousands of pages and includes complete detailed monograph of the product, the results of the trial, the proposed registered name of the product and the package insert. The data are reviewed by the drug control authorities and even by outside consultants who may require further information or clarification. If the documentation is accepted and is in compliance with the regulations, the drug control authorities can allow the drug to enter the market with the ‘New Drug Status’.

Phase IV (Post marketing surveillance/ studies): Once approval is obtained to market the drugs, phase IV of the trial begin. It is the post-licencing phase – field trials. The phase IV trial has no fixed duration as it is the surveillance phase during the post marketing clinical use of the drug. The performance of the drug is monitored for several years, immediately after marketing, to discover relatively rare side effects or previous unknown drug interaction of even previously known therapeutic use detected by a chance of discovery. During the ‘New Drug Status’ period, the manufacturers are expected to report any new information regarding the drug concerning its safety. The drug may remain in the ‘New Drug Status’  (i.e. controlled marketing) for several years until the drug control authorities are confident for its release to unrestricted marketing.