A drug development process has to pass through various stages to produce a drug product which is safe, efficacious, and has passed all drug regulatory requirements. Drug discovery program initiates because there is a medical problem for which there is no efficient treatment is available. The process of New Drug Development is explained in Figure 1.
THE NEW DRUG DEVELOPMENT PROCESS
Various stages of drug development include:
- Drug discovery
- Product characterization
- Formulation of drug product
- Preclinical studies
- Clinical trials
Phase of drug discovery start from target identification. Choosing a right target is essential for drug discovery, such as choosing a correct biochemical mechanism of a disease. The initial research on drug discovery usually occurs in academia or pharmaceutical industries. Drug candidates synthesized in research laboratory are tested for their interaction with drug targets. Usually, 5000–10,000 molecules are screened rigorously. This high number screening of molecules has been made possible with the advent of high throughput screening. Without high throughput screening it will not be possible to screen such high number of molecules in a day.
High throughput screening (HTS) can be defined as a process in which batches of potential leads are tested for their biological activity on the target. The leads obtained after screening of compounds through HTS are called as hits. These hits are obtained through automatized miniature robotic assay.
High Throughput Screening
A typical high throughput screening will contain seven components, namely:
- Target: A target can be defined as a substance which has key role in any biological process, for example, cyclooxygenase enzyme responsible for breakdown of cell arachidonic acid ‘a key factor in inflammation’ is drug target of NSAIDs like aspirin which acts on both COX 1 and COX 2 variants of cyclooxygenase to show its anti-inflammatory effect.
- Assay: An assay is usually performed to check the biological activity of the test compound with reference to its activity on the target. Fluorescence and radioactivity are generally used as indicators in the assay.
- A library of compounds should contain a wide range of structural diversity. Library size does not matter as compared to its diversity, quality and management.
- Information systems: Information systems are necessary for any type of data handling. These include resource scheduling, assay tracking, compound management, and data collection.
- Facilities: Facilities required are adequate laboratory space and environmental control.
Characterization of a potential drug candidate is done if it shows some promising therapeutic effects. Product characterization includes determination of shape and size of molecule, preferred storage conditions. Early pharmacological studies helps in determination of mechanism of action of the molecule. Moreover with the help of pharmacological studies we also determine toxicity and bioavailability of the compound.
Formulation, Delivery, and Packaging Development
A proper formulation of the drug molecule is required to ensure proper drug delivery. The drug product thus developed must be stable and easy to administer. If administration of drug product will not be simple then it will increase patient noncompliance. Therefore, it is a very critical stage in drug development as drug developer has to design a formulation which is stable, easy and safe to administer and look attractive.
With the help of pharmacokinetic study we determine parameters such as auc (area under the curve) which gives us the bioavailability of the drug, Cmax, i.e. maximum concentration of drug in the blood, Tmax, i.e. time required to reach the maximum concentration, T1/2, i.e. time in which half of drug is removed from the body, i.e. volume of distribution (Vd) of drug. Vd is the total volume in which the drug is distributed. Vd is a hypothetical volume and gives us information regarding the tissue binding of the drug. Sometimes Vd can be even larger than the total body volume.
Preclinical study is a risk based process which involves safety and efficacy evaluation of new drugs in animals. The results obtained from preclinical studies are extrapolated to obtain human outcomes. A new compound has to be tested for its safety and efficacy in animals before its use as a drug product in humans. The preclinical pharmacological and toxicological drug responses with respect to dosage form used, dosage regimen of the drug product and route of administration helps in initiating clinical studies on humans. Preclinical studies should be approved by the governing body (CPCSEA in India). Care must be taken while preclinical studies to avoid cruelty on animals and if animal has to be sacrificed it should be done duly under anesthesia.
International Conference on Harmonization (ICH) has issued guidelines outlining the technical requirements of acceptable preclinical studies of drug.
Preclinical studies are generally performed in two disciplines namely (1) general pharmacology and (2) toxicology.
Pharmacology is the study of effect of drug on the function of various body systems. There are various other factors along with the pharmacological action of a drug in a specific tissue at a particular receptor, which contribute in the success of drug therapy. When a drug enters the body processes of absorption, distribution, metabolism and excretion or elimination starts simultaneously. All these processes occur parallel. These are the processes of pharmacokinetics. Pharmacokinetics is what body does to the drug. The drug also acts on the body and shows its pharmacological and therapeutic action with the help of drugs binding to the receptor at tissue specific site. This is what drug does to the body and is called as ‘pharmacodynamics’.
Pharmacokinetics involves absorption, distribution, metabolism and excretion (ADME) of the drug, e.g. paracetamol is rapidly absorbed orally, attains peak plasma level in 30 to 60 minutes, 25% of drug is bound from plasma protein and remaining occur in plasma as free drug, Vd is 1 L/kg, extensively metabolized in liver and excreted in urine. Process of drug metabolism is explained in Figure 2.
Movement of drug from its site of administration to the central compartment or systemic circulation is called absorption. Rate and extent of absorption is termed as bioavailability. It is bioavailability about which clinicians are generally considered.
Figure 2: Absorption, distribution, metabolism, and excretion of a drugAbbreviation: GI, Gastrointestinal
When, a drug is administered orally, it is first absorbed from stomach and intestine, comes to liver through hepatic portal system, where it is metabolized extensively. This metabolism of drug into liver is termed as first pass metabolism or first pass effect. Along with the metabolism, biliary excretion of drug also occurs in liver before entering the drug into systemic circulation. If the metabolizing and excretory capacity of liver for a particular drug is comparatively higher, then it will greatly reduce the bioavailability of the drug, which may lead to therapeutic failure.
Following absorption, drug reaches in systemic circulation and is distributed to interstitial and intracellular fluid. This process of distribution, displays various physiological factors and physicochemical properties of the individual drug. Cardiac output, blood flow to the underlying area, blood vessel permeability and tissue volume govern the delivery rate and amount of drug distributed into tissues. Initially, drug is mostly distributed into well perfused organs such as liver, kidney and brain, whereas delivery to least perfused areas such as muscles, viscera, skin and fat is slower. Redistribution or second phase of drug distribution may take several minutes to few hours before the concentration of drug in tissues reaches equilibrium with that of in blood. Redistribution of drug involves larger fraction of body area in comparison to initial phase of drug distribution and generally accounts for most of the extravascular drug distribution. With, the exception of brain, drug diffuses readily into interstitial fluid because of the highly permeable nature of capillary endothelial membrane. Thus, tissue distribution is determined by the partitioning of drug between blood and particular tissue. Relative binding of drug to plasma proteins and tissue macromolecules is crucial in determination of drug distribution.
Lipophilic character of drugs promotes there passage through various biological membranes and there access to site of action. This lipophilic nature of drug serves as hindrance in excretion or removal of drug from the body. Only, a small fraction of drug is excreted unchanged through the kidneys, because lipophilic compounds filtered through glomerulus are largely reabsorbed into the systemic circulation. Thus, metabolism of drug is required to convert it into water soluble hydrophilic form, which is essential for elimination of drug from body as well as termination of their biological and pharmacological activity. Biotransformation or metabolism reactions are generally classified into:
- Nonsynthetic/phase 1/functionalization reaction
- Synthetic/phase 2/conjugation reaction.
Excretion of drug is its passage out from the systemic circulation. Drugs and drug metabolites are generally excreted in urine, feces, saliva and exhaled air, etc.
In pharmacokinetics, we have studied the action of body on the drug, while in pharmacodynamics we study the effect of drug on body. Pharmacodynamics involves study of biochemical and physiological effects of drugs and their mechanism of action.
Toxicological studies are performed to ensure safety of drug before its administration into human beings. Toxicological activity of a drug product is determined by in vitro and in vivo assay. In vitro studies are performed to examine the direct effect of drug on cell phenotype and proliferation. In vivo studies are performed for qualitative and quantitative determination of toxicological effects. LD50 values are also determined in toxicological studies.
Concept of LD50 (lethal dose 50) was first introduced in 1927 for establishing the toxic potency of biologically active compounds. LD50 value can be defined as ‘the statistically derived single dose of a substance which can cause death in 50% of the animals in an experimental group’. LD50 value of a drug can be determined using following methods:
- Fixed dose test: This test is useful in minimizing the number of animals required for LD50 value determination.Test procedure: Rats of either sex (preferably female) are dosed in a stepwise manner using the fixed doses of 5, 50, 300, and 2000 mg/kg (exceptionally an additional high dose of 5000 mg/kg may be also used). The initial dose level is selected as the dose expected to produce some toxicity other than mortality (if there is no information regarding initial dose and toxicity, initial dose will be 300 mg/kg). Animals may be further dosed of fixed doses at higher or lower level depending upon the presence or absence of signs and symptoms of toxicity in the animals.Observation: Animals are observed daily for 14 days. The animals are observed for changes in skin, fur, eyes and mucous membranes, respiratory, circulatory, autonomic and central nervous systems, and somatomotor activity and behavior pattern. Attention should be given to tremors, convulsions, salivation, diarrhea, lethargy, sleep and coma.
- Acute toxic class method: The test consists on a stepwise procedure with the use of three animals of either sex (normally females) per step. Absence or presence of compound-related mortality of the animals dosed at one step will determine the next step (i.e. no further testing is needed; dosing of three additional animals with the same dose; dosing of three additional animals at the next higher or lower dose level). The starting dose level is selected from one of four fixed levels (5, 50, 300, and 2000) and it should be that which is most likely to produce mortality in some of the dosed animals. When there is no information on a substance to be tested, it is recommended to use the starting dose of 300 mg/kg.
Observations are similar to first dose test.
- Up-and-down method: The test consists of a single ordered dose progression in which animals of a single sex (normally females) are dosed, one at a time. The first animal receives a dose step below the level of the best estimate of the LD50 (when no information is available to make a preliminary estimate of the LD50, the suggested starting dose is 175 mg/kg). If the animal survives, the dose for the next animal is increased by a factor of 3.2. Times the original dose; if it dies, the dose for the next animal is decreased by a similar dose progression.
Observations are also similar to fix dose test.
- Acute inhalation toxicity: This test provides information on health hazards likely to arise from short-term exposure by the inhalation route. Several groups of animals are exposed for a defined period to the test substance in graduated concentrations (at least three), one concentration being used per group. The preferred species is the rat and at least 10 animals (5 female and 5 male) are used at each concentration level. Animals should be tested with inhalation equipment designed to sustain a dynamic airflow of 12 to 15 air changes per hour, ensure adequate oxygen content of 19% and an evenly distributed exposure atmosphere. Where a chamber is used, its design should minimize crowding of the test animals and maximize their exposure to the test substance.Observations: Animals are observed daily for a total of 14 days. Observations include changes in skin and fur, eyes and mucous membranes, respiratory, circulatory, and autonomic and central nervous systems, and somatomotor activity and behavior pattern. Attention should be given to tremors, convulsions, salivation, diarrhea, lethargy, sleep and coma.
After conducting preclinical studies pharma companies submit data collected in preclinical studies to the Food and Drug Administration (FDA) and file NDA (New Drug Approval). If after 30 days of filing the NDA, there is no response from FDA then it is considered that the trial is not put on hold and companies can go ahead with the trials.
Clinical trials can be defined broadly as ‘the medical research involving the people’. Clinical trials are performed:
- To establish the safety of new drug or treatment in the human beings
- To determine its side effects
- To prove the efficacy of newer drug over the currently available one.
Phases of Clinical Trials
Clinical trials are generally classified into three phases according to their objectives. However, there is also a phase IV of clinical trials, which involves monitoring of drug, when it comes into market.
Phase 1 Trials
Phase 1 trials are the earliest use of a new medicine in the human beings. Phase 1 trials are also called as ‘clinical development or human pharmacology’ phase. Phase 1 trials are small trials recruiting less than thirty healthy human volunteers. Phase 1 trials help us to determine:
- Safe dose range
- Side effects of drug
- Adverse effects of drug if any
Phase 1 trials are classified into:
- Single ascending dose studies (SAD)
- Multiple ascending dose studies (MAD)
- Food effect
Single ascending dose (SAD) studies: In SAD studies a single dose of drug is given to the volunteers. The volunteers are kept under observation for a specific time and their blood samples are collected and assayed at regular intervals. If, volunteers do not exhibit any prominent adverse effect, then dose of drug is increased and is given to newer group of volunteers. This, process is continued until and unless we reach maximum safe level of the drug or intolerable side effects occur in the volunteers.
Multiple ascending dose (MAD) studies: In (MAD) studies small group of volunteers are given small multiple doses of a drug and are kept under observation. Their bloods samples are collected at regular intervals are analyzed. Dose of drug is increased for the next group.
Food effect: Food effect is a short trial which is designed to check the effect of food on the bioavailability of the drug.
Phase 2/Phase II Clinical Trials
After confirming the initial safety of drug in phase 1 trials, phase 21 trial is performed on large number peoples (200–300). In phase 2 of clinical trials effect of drug on the body is explored and phase 1 studies are continued in the phase 2.
Phase 2 studies are sometimes divided into phase 2A and phase 2B.
- Phase 2A or IIA is designed to prepare dosage regimen
- Phase 2B or IIB is designed to determine the efficacy of the drug.
Phase 3/Phase III Trials
Phase 3 trials compare the newer drug/treatment with the currently available standard treatment for a particular disease. Phase 3 trials involve large number of patients.
During the phase III studies, preparations are made for submitting the Biologics License Application (BLA) or the New Drug Application (NDA).
Phase 3 trials are generally multicentric so that the researchers can collect more information regarding drug and its effects in different environmental condition in a shorter duration. Phase 3 trials are generally randomized control trials. Randomization of trial means that the patients are grouped into two groups randomly. One group will receive standard drug and other will receive test drug.
Phase 4/Phase IV Trial
It is also called as post-marketing surveillance. It involves monitoring of safety data of drug, even after the drug obtains license for its sale. Several drugs have been withdrawn from market due to monitoring of their safety data in phase IV of clinical trials, e.g. Astemizole an H1 antagonist was withdrawn from the market after reports of severe cardiac toxicity, several coxibs (selective COX 2 inhibitors) were withdrawn from market on allegedly causing arrhythmia, gatifloxacin, a fluoroquinolone was withdrawn from market on causing photo toxicity.
Recently, a new phase, zero (Phase 0) has been added. Phase 0 trials are performed, while testing for new anticancer/anti HIV medicine in the cancer/HIV patients as these medicines cannot be used in normal individuals.