There is mounting evidence to suggest that immunization-based strategies can be used to mobilize the human immune system against specific
carbohydrate antigens displayed on the surface of
cancer cells. Following isolation and identification, such
antigens can be administered as
conjugate vaccines. The
tumor-associated
carbohydrate antigen KH-1 is 1 such
antigen and may serve as a potential target for immunization against
adenocarcinoma. However, a serious impediment to the application of a
vaccine-based approach involving this
antigen is that its availability from natural sources is severely limited. In order to overcome this limitation, we have developed an efficient total synthesis of this complex
glycolipid. We have extended our synthesis to reach a structurally related analog in which the
ceramide portion of KH-1 is replaced with an allyl substituent. These synthetic advances have led to the preparation of 2 potential
vaccine constructs, each based on the conjugation of the KH-1 nonasaccharide and the
carrier protein keyhole limpet hemocyanin (KLH). In 1 construct (KH-1-Et-KLH), the nonasaccharide is conjugated to KLH via a simple ethyl linkage, while in the other (KH-1-MMCCH-KLH), conjugation is mediated by a
4-(4-N-maleimidomethyl)cyclohexane-1-carboxyl hydrazide (
MMCCH) cross-linker. We report here the immunological properties of these 2 constructs. Mice were immunized with either of the 2 KH-1-KLH
vaccine candidates or the KH-1
ceramide, along with the
immunological adjuvant QS-21. Immunization with the
ceramide served as a negative control and, as expected, failed to stimulate the production of
antibodies against the KH-1
glycolipid. The construct in which the KH-1 nonasaccharide is linked to KLH via a simple alkyl chain stimulated significant quantities of
IgM antibodies, whereas the construct linked to KLH by
MMCCH induced high titers of both
IgM and
IgG antibodies. Inhibition data demonstrated that
antibodies generated in response to immunization with the KH-1-KLH constructs recognize not only the
KH-1 antigen but also the Lewis(y) (
Le(y)) antigen, which, from a structural perspective, is similar to the 4 residues located at the non-reducing end of the KH-1 nonasaccharide. Thus, the KH-1-KLH constructs elicit an immune response that successfully targets 2
adenocarcinoma markers. As assessed by FACS analysis, the
antibodies raised were strongly reactive with the KH-1/Le(y) positive cell line MCF-7 but not with KH-1 and Le(y) negative
melanoma cell lines. Based on the results of our study, a KH-1-KLH plus
QS-21 vaccine is being prepared for clinical evaluation.