Lydia A. Falk

Prelicensure Evaluation of Combination Vaccines

There is considerable public health interest in licensing safe and effective combination vaccines. Because combination vaccines may progress rapidly from phase 1 to a pivotal phase 2 immunogenicity trial, a rigorous approach to address product issues early in development is warranted. Clinical studies to evaluate the safety, immunogenicity, and (when necessary) clinical end point efficacy of combination vaccines should be randomized and well controlled in most cases. A large phase 3 safety study (i.e., a study that enrolls thousands of vaccinees) should be included in the development plan if a phase 3 (clinical end point) efficacy trial will not be conducted. Often, the new combination vaccine under development contains immunogens that have all been previously licensed, have demonstrated efficacy in earlier clinical trials, or both. For such products, comparative immunogenicity data may be sufficient to support efficacy. When applicable, clinical data to support simultaneous administration with other relevant vaccines should be obtained. Given the complexity of combination vaccine development, early consultation with United States Food and Drug Administration can be invaluable.

Evaluating the Immune Response to Combination Vaccines

Assessment of the immune responses to combination vaccines in the United States has generally been based on randomized, controlled comparative trials, with such studies designed to rule out predefined differences. In designing clinical studies of the immune response to combination products, attention should be directed toward selecting the appropriate immunologic end points and control groups. Acceptable differences in immune responses between combination and control groups should be predefined, and an adequate statistical plan should be developed. In many cases, it may be necessary to evaluate simultaneous administration of other recommended vaccines, assess schedule changes for 1 or more components of a combination, and bridge immunologic data obtained from international studies to the population of the United States. We discuss the use of immunogenicity studies to support the licensure of combination vaccines when field efficacy studies are either not possible or not required and highlight some recent experiences with combination vaccines containing Haemophilus influenzae type b polysaccharide conjugates.

Testing and Licensure of Combination Vaccines for the Prevention of Infectious Diseases

Vaccines are exceptionally cost-effective agents for infectious disease prevention, control, and (potentially) eradication. Combination vaccines can reduce the number of immunizations required to achieve protection against multiple diseases and consequently can lead to increased vaccine coverage (1,2). These advantages will become even more apparent as vaccines for new indications are approved (3,4). The current Advisory Committee on Immunization Practices (ACIP) recommendations already necessitate that a large number of vaccines be given to infants within a short time interval (5). The ACIP now recommends a sequential vaccination schedule of two doses of inactivated poliovirus vaccine (IPV) followed by two doses of oral poliovirus vaccine (OPV) for routine childhood vaccination (6), anticipated to be an interim step before the implementation of an “all IPV” policy in the United States. Thus, the advantages of combination vaccines are especially evident for pediatric populations.

Effects of Retinoic Acid on TGFβ Receptor Expression in HL‐60 Cells

HL-60, a human promyelocytic leukemic cell line, has been shown previously to undergo terminal granulocytic differentiation following treatment with the vitamin A derivative, retinoic acid (RA).’ The mechanism(s) involved in the reduced proliferative capacity and increased differentiation of HL-60 cells following RA treatment is (are) still unknown. Given the numerous studies showing the antiproliferative and differentiative effects of TGFPl on a variety of cell types: these studies were performed to examine the possible role of TGFPl in RA-induced differentiation of HL-60 cells. Treatment of HL-60 cells with RA (0.1 to 100 nM) for 7 days resulted in a dosedependent decrease in basal proliferation and a concomitant increase in differentiation as determined by [3H]thymidine incorporation and morphologicallcytochemical analysis respectively. High concentrations of RA (100 nM) resulted in an increase in both mature granulocytes (40%) and nitroblue tetrazolium positive cells (58 %). The addition of exogenous TGFPl alone had little effect on the proliferative and differentiative capacity of HL-60 cells. However, at suboptimal concentrations of RA (1.0 nM), the addition of exogenous TGFpl resulted in a marked decrease in proliferation with no effect on differentiation (FIG. 1). To determine whether the enhanced antiproliferative effect observed in the presence of X F p l and suboptimal concentrations of RA was due to modulation of TGFP receptors by RA, receptor expression was examined by chemical crosslinking analysis using 1251-labeled TGFpl. Untreated HL-60 cells showed little TGFP receptor expression. However, treatment with RA resulted in a dose-dependent increase in receptor expression with maximal receptor expression obtained with 10 nM RA. This increase in receptor expression was seen predominantly in the 65 kD binding protein (FIG. 2). In addition to effects on TGFP receptor expression, we also examined the effects of RA on the expression of TGFP mRNA. Total RNA was isolated from HL-60 cells treated with or without RA for 7 days and analyzed by Northern blot. Untreated HL60 cells exhibited low levels of TGFP mRNA species, which were increased following RA treatment. Maximal induction of TGFP mRNA was seen in cells treated with 10 nM RA. In conclusion, combined treatments of HL-60 cells with E F p l and suboptimal concentrations of RA resulted in a synergistic antiproliferative, but not differentiative,