Immunostimulation. Clinical and experimental perspectives.
Three classes of immunostimulating drugs are described, each representing a different approach to the problem of pharmacological immunostimulation. The rationale for the use of microbes or microbial agents as immunostimulators rests on the fact that some micro-organisms, especially those that replicate intracellularly, carry a special potential to activate macrophages. Clinically, the use of these agents in patients with tumors and infections has been disappointing; however, there have been positive exceptions like the responsiveness of melanomas and bladder carcinomas to the injection of BCG. Many of the inconclusive results may be due to insecurities in the dosage of microbial preparations and to a general lack in standardization. Some structures with high efficacy and low toxicity which have recently evolved from this field deserve further investigation. A number of structurally unrelated synthetic compounds was found to influence immune parameters. Levamisole can today be classified as an immunostimulating drug with limited utility in recurring infections and in chronic polyarthritis. Several immunostimulating drugs which have attracted interest contain a purine as the effective component. This is not surprising in view of the fact that many genetically determined immunodeficiencies can be traced to defects of enzymes which play a crucial role in purine biosynthesis. Finally, the potential role of lymphokines as stimulators of the immunosystem is briefly described. Some of these glycoproteins have recently become available for clinical trials. Others will be made available through genetic engineering. The therapeutic utility of these compounds is not yet clear; they will, however, be of great value as probes for the study of immune functions and for the development of immunopharmacology.