The Manufacture and Quality Control of Immunological Products

THE MANUFACTURE AND QUALITY CONTROL OF IMMUNOLOGICAL PRODUCTS

INTRODUCTION

Immunological products comprise a group of pharmaceutical preparations of varied composition but with a common pharmacological purpose: the modification of the immune status of the recipient, either to provide immunity to infectious disease, or in the case of in vivo diagnostics, to provoke an indication of immune status usually signifying previous exposure to the sensitizing agent. The immunological products that are currently available are of the following types: vaccines; in vivo diagnostics; immune sera; human immunoglobulins; mono-clonal antibodies; and antibody-targeted therapeutics and diagnostics. For the purpose of this chapter, cell biology-derived immunomodifiers with a non-specific action, e.g. cytokines or chemokines, are not included.

Vaccines are by far the most important immunological products. They have enabled the control or eradication of numerous infectious diseases affecting humans and their domesticated animals. For example, the systematic application of smallpox vaccine, deployed under the aegis of the World Health Organization (WHO), achieved the eradication of one of the most devastating infections. Similarly, the universal application of poliomyelitis vaccine has brought poliomyelitis to the verge of eradication. Diphtheria, tetanus, pertussis (whooping cough), measles and rubella vaccines have been applied worldwide through national or UNICEF-sponsored healthcare programmes and have virtually eliminated these diseases in those countries in which there have been the resources and the will to deploy them effectively. Vaccines that provide protection against many other infections are available for use in appropriate circumstances. Some, such as hepatitis B and conjugate vaccines against Haemophilus influenzae b (Hib), meningococci and pneumococci, have had a huge impact on morbidity and mortality wherever they have been applied.

The range of disorders that may be prevented or treated by vaccines has enlarged considerably beyond infectious diseases. Vaccines are currently undergoing evaluation for several other purposes including therapy of cancer; prevention of allergies; desensitization of allergic patients; fertility control; and treatment of addictions.

In vivo diagnostics such as tuberculins, mallein, histoplasmin, coccidioidin and brucellin, are used to demonstrate an immune response, and hence previous exposure, to specific pathogens as an aid to diagnosis. Allergen skin test diagnostics are used to indicate sensitization to materials of biological origin that may be present in the environment or in specific products. Others, such as the Schick test (diphtheria) toxin are used to detect the presence of protective immunity. Because of their clinical and pharmaceutical limitations, the trend has been to phase out these preparations, and tuberculins (as purified protein derivative, PPD) are now by far the most important of this group.

Immune sera, which were once very widely used in the prophylaxis and treatment of many infections, have more limited use today. Vaccines and antibiotics have super-seded some and lack of proven therapeutic benefit has caused others to be relegated to immunological history. However, some still play an important role in the management of specific conditions. Thus, diphtheria and botulinum antitoxins prepared in horses remain the only specific treatments for diphtheria and botulism respectively. Equine tetanus antitoxin is still used as an effective prophylactic in some parts of the world, although largely replaced by human tetanus immunoglobulin in developed countries. Similarly, antivenins prepared in horses, sheep, goats or other animals against the venoms of snakes, spiders, scorpions and marine invertebrates still provide the only effective treatment for venomous bites and stings and are important therapeutic agents in some parts of the world.

Human immunoglobulins have important but limited uses, for example in the prophylaxis of hepatitis A, hepatitis B, tetanus and varicella zoster. Additional specific immunoglobulins against diphtheria and botulism toxins are under development and vaccinia immunoglobulin may be reintroduced. Monoclonal antibodies to bacterial endotoxin, to cytokines involved in the pathogenesis of septic shock and to specific infectious agents have been developed and evaluated clinically but have yet to enter into general use. Monoclonal antibodies against specific cell receptors have undergone a rapid development and are employed successfully in cancer therapy and are under development for treatment of autoimmune disease. Immune sera and human immunoglobulins depend for their protective effects on their content of antibodies derived, in the case of immune sera, from immunized animals and, in the case of immunoglobulins, from humans who have been immunized or who have high antibody titres as a consequence of prior infection. The form of immunity conferred is known as passive immunity and is achieved immediately but is limited in its duration to the time that protective levels of antibodies remain in the circulation.

Vaccines achieve their protective effects by stimulating the immune system of the recipient to produce T-cells and/or antibodies that impede the attachment of infectious agents, promote their destruction or neutralize their toxins. This form of protection, known as active immunity, develops in the course of days following infection and in the case of many vaccines develops adequately only after two or three doses of vaccine have been given at intervals of days or weeks. Once established, this immunity can last for years but it may need to be rein-forced by booster doses of vaccine given at relatively long intervals. The immunogenicity of some vaccines can be improved by formulating them with adjuvants. These are a heterogeneous group of substances which enhance the immune response. Aluminium hydroxide gel (hydrated aluminium oxide) and aluminium phosphate are the only ones currently in general use in human vaccines. A much wider range of substances including oily emulsions, saponin, immune-stimulating complexes (ISCOMS), monophosphoryl lipid A, CpG motif contained in oligo-deoxynucleotide (CpG-ODN) and others are used in veterinary vaccines, and some are under investigation for use in human vaccines.

Different types of infectious agent require preferential mobilization of different arms of the immune response. For example, toxigenic bacterial infections require the production of toxin neutralizing antibodies whereas intracellular bacterial infections such as tuberculosis require cell-mediated responses involving mixed T-lymphocytes and activated macrophages, whereas many viral infections will require neutralizing antibody and cytotoxic T-cell responses for effective protection. Achieving the appropriate response can be difficult and in the past has had to be approached empirically. This is why most successful viral vaccines have been based on live attenuated strains, which simulate natural infection. Non-living vaccines have been effective against many bacterial infections but markedly less so against those requiring sustained cell-mediated responses. The development of more selective vaccine adjuvants and delivery systems promises to put the future process of vaccine design on a more rational basis.

A property common to vaccines, immune sera and human immunoglobulins is their high specificity of action. Usually each provides immunity to only one infection, although in some cases cross-protection can occur, e.g. BCG protects against both tuberculosis and leprosy. Where it is necessary to protect against more than one type of agent, monospecific preparations can be combined. For example, botulism antitoxin usually covers types A, B and E; meningococcal polysaccharide vaccine may cover groups A, C, W125 and Y; and pneu mococcal polysaccharide vaccine usually covers 23 sero-types. Heterologous preparations may also be combined as in measles/mumps/rubella and diphtheria/tetanus/pertussis vaccines. With the increasing number of vaccines for infants and young children, the trend is to produce more complex combinations such as diphtheria/tetanus/pertussis/hepatitis B/inactivated polio/Hib vaccine, to minimize the number of injections. The possible additive or interactive effects of the various components on the immune system have raised concerns about the safety of such combinations. While some evidence of reduced responses to certain components has been obtained, there is little to support suggestions of serious adverse effects from current combinations.

In addition to the three main types of immunological products that are widely available, more specialized preparations include: synthetic peptide immune response modifiers such as those used to block T-cell responses in multiple sclerosis; monoclonal antibodies for cancer therapy or diagnosis; and hybrid toxins containing a bacterial or plant toxin subunit attached to an antibody or human cell receptor-binding protein, and also intended mainly for cancer therapy. These have rather limited applications and for the most part, are designed to suppress or exploit the specificity of immune responses rather than to stimulate them.