This study describes the safety and immunogenicity of a liposome-based vaccine injected into human subjects. Thirty healthy adult male volunteers were immunized with a liposome-encapsulated recombinant protein (R32NS181) containing epitopes from the repeat region of the circumsporozoite protein of Plasmodium falciparum. This antigen had previously been found to be poorly immunogenic in humans when it was adsorbed with Al(OH)3. In the present study, R32NS181 was encapsulated in liposomes containing monophosphoryl lipid A that were subsequently adsorbed to Al(OH)3. Increasing doses of liposomes containing antigen and monophosphoryl lipid A were used, but the liposomes were always adsorbed to the same dose of Al(OH)3. R32-specific serum IgG antibody responses to liposome-encapsulated R32NS181 were much higher than levels attained previously in humans with R32NS181 adsorbed to Al(OH)3. Geometric mean specific IgG levels after three doses ranged from 14 to 33 pg/ml. Sera from volunteers receiving the two highest doses inhibited P. falkiparum sporozoite invasion of cultured hepatoma cells by an average of 92%, a result that was again superior to previously reported vaccines. Moderate but acceptable transient local reactogenicity was noted at high doses of the vaccine formulation, but little or no systemic toxicity was seen despite liposomal monophosphoryl lipid A doses up to 2200 pug.We conclude that encapsulation of poorly immunogenic circumsporozoite protein repeat peptides in monophosphoryl lipid A-containing liposomes is a successful adjuvant strategy in humans for inducing high levels of specific antibody production.Practical development of modern vaccines has been greatly advanced by the availability of synthetic antigens, but progress has been hindered in some cases by poor immunogenicity ofthe antigens. Liposomes have been successfully used as drug carriers (1) Although some protective immunity against experimental P. falciparum challenge was achieved after immunization of humans with an Al(OH)3-adsorbed recombinant protein (7) or a peptide-tetanus toxoid conjugate (7) containing repeat epitopes, the immune response was poor (7-9).In an effort to improve the humoral immune response, we designed and tested in animals a variety of Al(OH)3-adsorbed candidate liposomal vaccines containing lipid A or monophosphoryl lipid A (MPLA) and repeat-based malaria antigens (10-13). The success of this approach in animals resulted in initiation of the present clinical trial. The recombinant antigen used in the current study (R32NS181) has also been recently evaluated in humans with other adjuvant systems that were more effective than Al(OH)3 but less potent than the liposome formulation used in the present study (14). MATERIALS AND METHODSAntigen. The antigen, designated R32NS181 (also called R32NS1), was expressed in Escherichia coli and purified by SmithKline Beecham Pharmaceuticals. It consists of a fusion protein with the amino acid sequence MDP[(NANP)15-(NVDP)]2NS181 (13). NS181, which refers to a sequence of 81...

Seventeen malaria-naive volunteers received a recombinant Plasmodium falciparum vaccine (RLF) containing the carboxy- and the amino-terminal of the circumsporozoite protein (CSP) antigen without the central tetrapeptide repeats. The vaccine was formulated in liposomes with either a low or high dose of 3-deacylated monophosphoryl lipid A (MPL) and administered with alum by intramuscular injection. Both formulations were well tolerated and immunogenic. MPL increased sporozoite antibody titers measured by ELISA, Western blot, and immunofluorescence assay. One high-dose MPL vaccine formulation recipient developed a CSP-specific cytotoxic T lymphocyte response. After homologous sporozoite challenge, immunized volunteers developed patent malaria. There was no correlation between prepatent period and antibody titers to the amino- or carboxy-terminal. The absence of delay in patency argues against inclusion of the amino-terminal in future vaccines. A significant cytotoxic T lymphocyte response may have been suppressed by the inclusion of alum as an adjuvant.

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