MENÚ PRINCIPAL

Four main antibody characteristics are thought to influence the efficacy o f tumour targeting: molecular weight, affinity, avidity and specificity. Specificity, affinity and avidity are interrelated as they describe the interaction between the antibody and the target antigen. An antibody is specific if it recognises an epitope on the antigen. The affinity describes the strength o f binding between the antibody and antigen. Avidity also describes the binding strength and takes into account the valency o f the interaction. Therefore, avidity is always equal to or greater than affinity.

Antibody engineering technology facilitates the manipulation o f these characteristics which allows the investigation o f their effect on tumour targeting. However, it has proved difficult to investigate the role o f each characteristic in isolation which has resulted in conflicting evidence and uncertainty over the cause o f any observed biological effect. For example, some evidence suggests that antibodies with higher affinity (Adams et al., 1998) and avidity (Schlom et al., 1992; King et al, 1994) improve tumour targeting while others postulate that they create a "binding-site barrier' around blood vessels that prevents tumour penetration (Weinstein et al., 1987 &1992; Fujimori et al, 1990). In addition, increasing valency also increases the molecular weight which reduces the clearance rate and subsequently increases tumour uptake and normal tissue toxicity. Consequently, the relative importance o f each characteristic is not fully understood. For this reason, the manipulation o f these

characteristics has yet to significantly improve the tumouricidal effect o f targeted therapy.

Compounding these issues, many studies have used inappropriate measures o f therapeutic efficacy. The principal biological parameter that ultimately determines therapeutic efficacy is the surviving cell fi’action after treatment. For any radiation therapy, the tumour response depends on the absorbed dose and its pattern o f delivery, the radiosensitivity o f cells, repair capacity and rate o f proliferation o f cells. In RIT, there is a large temporal variation in dose rate with prolonged treatment times. Therefore, the effect o f dose-rate and proliferation are o f

particular importance. However, many assessments o f therapeutic efficacy have ignored some o f these factors and have used physical measurements as predictors o f biological outcome. This has added further confusion to the search for the optimal targeting molecule.

A technique often used to assess the therapeutic efficacy o f radiolabelled antibodies involves the use o f biodistribution data (Lane et al., 1994; Casey et al., 1996) in humans and animals to calculate the absorbed dose in tumour and normal tissues. The tumour to normal tissue ratio was then used as a measure o f the therapeutic efficacy. However, this technique ignored the effects o f dose rate and proliferation during treatment.

Another common experimental technique uses a biological measurement to assess the efficacy o f radiolabelled antibodies in animal models. In this case the extent o f tumour growth inhibition is determined by measuring the tumour size after a therapeutic dose o f radiation (Pedley et al., 1993). This can be subject to large errors due to the difficulty in obtaining a sufficient number o f similarly size tumours to give statistically significant results. In addition, it does not allow the correlation o f any physical parameters such as the absorbed dose, in tumour and normal tissues, with biological effect. Furthermore, there m ay be significant cross-dose fi*om tissues surrounding the tumour causing further growth inhibition.

In recent years, compartmental models have been developed to study the relationship between individual antibody parameters and antibody pharmacokinetics (Thomas et al, 1989; Strand et al, 1993; Baxter, 1994). The construction o f these models is only possible by reducing a complex system to a series o f compartments where the relationship between each

compartment can be described in mathematical terms. These models can provide considerable insight into how antibody properties affect tumour uptake and pharmacokinetics, and can facilitate an improved understanding o f difficult concepts involving many factors. However, they have yet to be used to relate these physical parameters to biological effect.

One characteristic that is known to have a large effect on antibody biodistribution and pharmacokinetics is the molecular weight. Essentially, this determines whether the antibody is extracted fi*om the circulation by the kidney glomerulus, which ultimately governs the rate o f clearance and therefore the normal tissue toxicity and tumour localisation. According to mathematical pharmacokinetic models, tumour localisation relies on an antibody

concentration gradient from the blood to the tumour. Initially, the gradient is high with efficient tumour loading. As the antibody clears from the blood, the gradient gets smaller and tumour loading decreases until the gradient reaches zero. At this point, the gradient reverses and the amount o f antibody in tumour decreases. Consequently, antibody fragments that clear quickly give both lower tumour and normal tissue levels and subsequently reduce normal tissue exposure. In contrast, large antibodies show good tumour localisation but also give more exposure to normal tissues. This is suggested by the finding that small and large antibodies give similar tumour levels when renal excretion is eliminated by

nephrectomisation (Horal et al., 1998). Therefore, one way to optimise targeted therapy is to find a balance between clearance and localisation.

Assuming no immune reaction, the rate o f clearance is determined primarily by the molecular weight o f the antibody although molecular shape and charge are also known to affect

filtration (Lote, 1992). Small antibodies, less than 25 kD, are extracted by the glomerular filter in the kidney. As the antibody size increases, so does the restriction to filtration

(Sumpio & Hayslett, 1985). Larger antibodies are generally cleared by the reticuloendothelial system (RES), mainly via the liver and spleen.

In this chapter, a comparison o f antibodies o f different m olecular weight, affinity, valency and specificity is described in terms o f clearance from blood, biodistribution and total dose delivered to tumour and normal tissues. In addition, the effect o f dose rate and tumour cell proliferation during treatment is assessed. The principal purpose is to develop a more realistic measure o f the biological effect o f radiolabelled antibodies. A statistical model is then applied to the experimental data to address the complex nature o f the inter-relationships o f several factors in RIT and to assess their importance for effective therapy.