Phase 1 Study of the Safety and Immunogenicity of a Malaria Transmission-blocking Pfs25-Pfs25 Conjugate Vaccine

Malaria is a disease affecting millions of people in Africa, Asia, Central and South America
. It has its highest frequency, morbidity, and mortality in infants and young children. For
many reasons, a vaccine that would prevent this disease is sought.

Plasmodium falciparum and Plasmodium vivax, the most common and severe forms of malaria,
have host and stage specific proteins that can induce immunity to disease caused by this
pathogen. Surprisingly, vaccines against stages that infect mosquitos will prevent the
spread of malaria (transmission-blocking). We have developed a vaccine composed of a single
protein, denoted as Pfs25, to induce transmission-blocking antibodies in the human host.
These transmission-blocking antibodies will be ingested by the mosquito and inactivate the
sexual forms of the plasmodia (ookinetes) as they develop in the midgut of the mosquito.
Because Pfs25 is present only in the mosquito, humans do not develop antibody to this
antigen even in endemic areas. No antibodies were detected against Pfs25, a 21-kD protein
expressed on zygotes and ookinetes of P. falciparum and known to be a sensitive target of
transmission-blocking antibodies. Theoretically, frequent boosting with this antigen, as is
required for the other experimental malarial vaccines, would be necessary.

Our investigational vaccine is based upon the following:

1. Pfs25 is a low molecular weight protein and a poor immunogen.

Bharti et al showed the protective action of antibodies to Pvs25 raised by intensive
immunization of rabbits and mice (these regimes are not suitable for humans) to the
recombinant protein. These findings have been confirmed;

2. Transmission of Plasmodia to mosquitos is mediated by their ingestion of gametocytes in
the human bloodstream.

Within 10 minutes, these gametocytes transform into gametes in the midgut of the
mosquito. The male gametocyte fertilizes the female to form a zygote. Twenty four hours
later ookinetes pierce the epithelium of the midgut and differentiates into an oocyst.
This cell type forms the sporozoite that lodges in the salivary gland and is the
infective form injected by the mosquito into the human host. Pfs25 may also be involved
in survival of the ookinete in the midgut and its transformation into the oocyst;

3. The immunogenicity of poor immunogens is increased, especially in infants, by
covalently binding them to medically-acceptable carrier proteins to form conjugates.

The Pfs25 was bound to itself by the synthetic scheme used for Haemophilus influenzae type b
and other polysaccharides to proteins. The resultant conjugate was immunogenic and,
unexpectedly, antibodies elicited by this vaccine continued to rise 3 months and 7 months
after the second and third injections. The property of the Pfs25 conjugate to elicit
long-lived antibody at high levels is unique to this construct. Our data from Pfs25
conjugate in mice suggest that administration of this vaccine to the whole population
including infants along with their routine immunizations may be sufficient to elicit
long-lived, effective transmission-blocking activity.

Our plan is to evaluate clinical lots of this conjugate for P. falciparum and for P. vivax,
for their safety and immunogenicity in adults, children and infants, and then to establish a
clinical site to test their efficacy.

- ELIGIBILITY AND EXCULUSION CRITERIA

Healthy adults, 18 to 49 years of age of either sex who do not have any of the following
conditions will be eligible to participate:

1. A chronic or progressive disease requiring chronic medication,

2. History of surgical splenectomy or abnormal immune system,

3. History of neurological symptoms or signs, or mental illness,

4. Anaphylactic shock following administration of any vaccine or any other severe
allergic reaction.,

5. Women who are pregnant or intend to become pregnant during the vaccine study,

6. Had malaria or received a malaria vaccine previously,

7. Allergy to vaccine components or to nickel and yeast,

8. Had cancer, HIV/AIDS, Hepatitis B or C, Guillain Barre Syndrome, chronic skin disease
or have abnormal liver functions or blood counts.