Drug Discovery and Development Processes

INTRODUCTION

The human body is a miracle but it is also extremely vulnerable. Many illnesses and disorders are still untreatable. Fortunately science is always evolving. It is unravelling more and more secrets about how our body works and which process is occurring in conditions of sickness or health. The challenge is to use our scientific knowledge to discover new, innovative drugs, a new hope for the patients all over the world.

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Drug Designing or Drug Discovery and Development is an inventive process of finding new medications based on understanding of the biological target. Vast majority of drugs are small molecules designed to bind, interact and modulate the activity of specific biological proteins. These proteins which may also be receptors bind to and interact with other molecules to perform the numerous functions required for the maintenance of life. In many illnesses, one or more proteins or receptors in the body are not working correctly. That is what the scientists try to detect. If they discover which proteins or receptors cause an illness then these same proteins become the target for the development of a new drug. Thus, the target is the naturally existing cellular or molecular structure involved in the pathology of interest that the drug-in-development is meant to act on. Drugs work by interacting with target molecules (receptors) in our bodies and altering their activities in a way that is beneficial to our health. In some cases, the effect of a drug is to stimulate the activity of its targets (an agonist) while in other cases the drug blocks the activity of its target (an antagonist).

DRUG DISCOVERY PROGRAMME

A drug discovery programme initiates because of a disease or a clinical condition for which a suitable medical product is not available. It is this unmet clinical need which is the underlying driving motivation for the project. Developing a drug from an original idea to the launch of the finished product is a very labor-intensive, time consuming and expensive procedure which can take over 14 years to complete.

Target-based drug discovery starts with a thorough understanding of the disease mechanism and the role of enzymes, receptors or proteins within the disease pathology. The initial research would include experimental procedures to identify proteins responsible of the disease and generation a hypothesis that the inhibition or activation of those protein or pathway will result in a therapeutic effect in a disease state. The outcome is the selection of a target which may require further validation prior to progression into the lead discovery phase in order to justify a drug discovery effort. During lead discovery an intensive search ensues with the help of a technique called High Throughput Screening (HTS) to find a drug-like molecule or biological therapeutic, typically termed as a development candidate, that will progress into the preclinical, and if successful, into clinical development and ultimately be a marketed medicine.

Drug discovery process from target identification and validation through to filing of a compound and the approximate timescale for these processes. FDA: Food and Drug Administration; IND: Investigational New Drug; NDA: New Drug Application.

DRUG TARGETS

One of the most important steps in developing a new drug is target identification and validation. A target is a broad term which can be applied to a range of biological entities such as proteins, genes and RNA. A drug target is a key molecule involved in a particular metabolic or signal transduction pathway that is specific to a disease condition or a specific disease. Knowing the cellular targets of drugs is crucial if the process of drug discovery is to be made more efficient. Identifying the full spectrum of targets associated with a bioactive small molecule can lead to faster optimization, understanding of off-target side effects and the ability to minimize possible toxicities early on in the process. It is vital to have as much evidence as possible to support a target of choice before investing more resources in the target.

Good targets share several features: involvement in a crucial biological pathway; distinction from any previously known target; functionally and structurally characterized; and druggable. A ‘druggable’ target is accessible to the putative drug molecule and upon binding elicits a biological response which can be measured both in vitro and in vivo. It also needs to be efficacious, safe, meet clinical and commercial needs. When searching for novel drug targets, candidates can be assessed according to how many of these features they have, as well as participation in a biological process critical to the disease. Identification of the target is followed by its validation which a process of physiologically, pathologically and pharmacologically evaluating a biomolecule. It might be performed at the molecular, cellular or whole animal level. The potential drug target is then subject to high-throughput screening against a library of drug-like compounds or to rational drug designing.

However, the term ‘drug target’ itself has several limitations. The following points should be kept in mind: First, a drug is disease-dependent, that is, every target is involved in a spectrum of diseases. Second, most human diseases are rather complicated and involve a number of risk factors, so there clearly are many different targets with respect to a specific disease. Targeting a specific target could not conceivably cure a disease. Third, there are many drugs targets the same target and one drug may have more than one target. The relationship between a drug and its target is not one-to-one but many-to-one.

According to whether there are drugs available, a drug target can be classified into two classes: established drug targets and potential drug targets. The former are those for which there a good scientific understanding, supported by a lengthy publication history regarding both how the target functions in normal physiology and how it is involved in human pathology. Furthermore, there are many drugs targeting this target. The latter are those biomolecules whose functions are not fully understood and which lack drugs targeting them. Potential targets suggest directions for complete new drug research.

At present, the most frequent protein targets for which successful drugs have been developed include proteases, kinases, GPCRs and nuclear hormone receptors. GPCRs and enzymes represent the most important classes of proteins for drug discovery. According to the DrugBank database, there are 435 effect-mediating drug targets in human genome. These structures are targets of 989 unique drugs, through 2,242 drug-target interactions. The dataset shows that receptors make up the largest group of drug targets: 193 proteins (44%) of the human drug targets) are receptors, and 82 (19%) of these are G protein-coupled receptors (GPCRs). In overall dataset, ~36% of drug targets are GPCRs. Ligand-gated ion channels are second largest receptor target class followed by receptor tyrosine kinases at the third place.

Enzymes are the second largest group of target proteins in the human genome, comprising 29% of all human drug targets. Hydrolases are the most common class of enzymatic drug targets, comprising 49% of all human enzyme drug targets followed by oxidoreductases and transferases comprising 27% and 19% respectively. In addition, the majority (78%) of the enzyme targets are soluble proteins and not membrane-associated proteins. E.g.; cyclooxygenase 1 and cyclooxygenase 2, which belong to the oxidoreductase family are targeted by acetylsalicylic acid (aspirin).