NIEHS Awards Grants on Health Effects of Ozone

Exploring the fundamental mechanisms behind the low tumour uptake of labelled monoclonal antibodies (MoAbs) during in vivo immunotargeting, experiments were performed to estimate the in vivo value of the association constant (Ka) in an experimental targeting reaction. An artificial tumour model was utilised, based on diffusion chambers (DC) filled with antigen-coated polymer particles, implanted i.p. in normal, immunocompetent mice (NMRI/BOM). The MoAb H7 with specificity for placental alkaline phosphatase (PLALP) was chosen for this experiment. Each mouse carried two DC, one target DC filled with PLALP-coated particles, and a second control DC with the same amount of uncoated particles. The DC contained escalating doses of particles, ranging from 0.1 mg to 16 mg per DC, with groups of 6-12 animals per dose level. The next day after the implantation, a constant dose of '25l-labelled Fab fragments of H7 was injected i.v. in each mouse. The association constant Ka as measured from the binding data obtained in vivo was not significantly different from the value measured in vitro when the same target DC were incubated with the '25l-Fab in test tubes. This indicates that in vivo impairment of the antibody avidity is not the reason why a relatively low tumour uptake is generally experienced in immunotargeting studies. With the development of the hybridoma technology for production of monoclonal antibodies (Kohler & Milstein, 1975), there has been an increasing interest in diagnosis and treatment of cancer with labelled antibodies to tumour associated antigens (Epenetos & Kosmas, 1989). However, for various reasons, the tumour uptake of labelled MoAbs is disappoin-tingly low in most targeting studies. Firstly, non-specific binding of antibody in normal tissues is regularly experienced. Secondly, the antibody concentration in the tumour is lowered due to antibody catabolism and clearance. Thirdly, various local mechanisms such as tumour vas-culature, blood flow, tumour tissue permeability, and the tumour interstitial pressure hampers the antibody access to the tumour antigen (Jain, 1989; Matzku et al., 1990). Moreover, as in every immunological reaction, the avidity of the antibody should presumably be a factor of uttermost importance to the tumour uptake. If the avidity is impaired in vivo, this would contribute to the low tumour uptake. To be able to explore this question, experimental binding data from in vivo targeting reactions where the amounts of antibody and antigen are under control is required. This problem was overcome by applying the artificial tumour model elaborated by us (Fjeld et …