The use of biotechnology has led to the creation of novel classes of drugs that have transformed therapeutic approaches for various diseases that were otherwise difficult to obtain with traditional small molecule drug development. Monoclonal antibodies (mAbs), peptides and RNAi are therefore engineered and have required different approaches with their development (to determine efficacy, pharmacokinetics, safety) into the clinic. By far the greatest class of biologicals are mAbs (>50 having clinical acceptance, and hundreds more being developed).
Proteins and Polypeptides and Gene Therapy
Proteins and endogenous molecules have been used for many years as therapies; some examples are insulin, factor VIII and human growth hormone. They include first generation proteins that are copies of endogenous proteins or antibodies, sometimes produced by recombinant DNA technology. However difficulties in extraction, the use of animal hormones to evoke an immune response and the danger of transmission of infectious agents (e.g. Creutzfeldt-Jakob disease) led to second generation technologies which were engineered to improve on these limitations ( Table 8.1 ). Third generation agents are macromolecules where the structure is completely novel and the result of a biochemical design process. An example is mipomersen, an antisense RNA product that binds to mRNA coding for apolipoprotein B100, in the treatment of familial hypercholesterolemia.
| Type of change | Protein | Indication | Reason for Change |
|---|---|---|---|
| Altered amino acid sequence | Insulin | Diabetes | Faster-acting hormone |
| Tissue plasminogen activator analogues | Thrombolysis | Longer circulating half-life | |
| Interferon analogue | Antiviral | Superior antiviral action | |
| Factor VIII analogue | Haemophilia | Smaller molecule, better activity | |
| Diphtheria toxin-interleukin-2 fusion protein | T-cell lymphoma | Targets toxin to appropriate cells | |
| Tumour necrosis factor receptor-human immunoglobulin G Fc fusion protein | Rheumatoid disease | Prolongs half-life | |
| Altered carbohydrate residues | Glucocerebrosidase enzyme | Gaucher’s disease | Promotes phagocyte uptake |
| Erythropoietin analogue | Anaemia | Prolongs half-life | |
| Covalent attachment to polyethylene glycol | Interferon | Hepatitis C | Prolongs half-life |
| Human growth hormone | Acromegaly | Prolongs half-life |
Gene therapy involves the transfer of recombinant nucleic acid into cells to cause genetic modification to prevent, alleviate or cure disease. It has potential application and requires the use of vectors (e.g. viral or non-viral) to deliver DNA.
Antibodies
Pharmacology of antibodies
The very nature of antibodies means that they will have exquisite affinity and selectivity for a target protein. Thus, off-target effects are rarely observed, which can make toxicology studies less demanding. However the predicted on-target effects have to be carefully considered. They may have multiple modes of action relating to the target that exhibits non-linear log dose-response curves ( Fig. 8.1 ). MAbs often have a single optimal biological dose and this creates a biological response that is quantal rather than graded (i.e. a very steep dose-response relationship) due to mAb affinity for a target, instead of the proportional effects that are more accustomed to the small molecule drug-receptor relationships. Therefore traditional Scatchard plots that assume only a K d variable have to be replaced by non-linear algorithms.
Consideration has to be made of the sheer size of antibodies (human IgG is 150,000 daltons) compared to small molecules (around 150 daltons). However the architecture of antibodies can be manipulated so that certain characteristics of the protein structure are modified. Therefore the size of these fragments can be varied ( Fig. 8.2 ). Administration tends to be limited to intravenous or depot routes. Generally, whole antibodies do not internalize into cells, nor are they able to pass the blood–brain barrier. Therefore target selection for antibodies has to be made with these constraints in mind, although new technologies are being creating to engineer antibody fragments that can circumvent these constraints.
