Adaptación recíproca

Each absorbed dose calculation was normalised so that blood received the same dose for each antibody/radionuclide combination. The highest tumour to blood ratios were achieved for the specific antibodies with intermediate clearance rates. This supports the idea that there should be an adequate concentration gradient fi-om blood to tumour to ensure good tumour

localisation, while normal tissue sparing relies on rapid clearance. Finding a balance between tumour localisation and normal tissue clearance can improve RIT.

The lowest ratios o f dose in tumour to blood were obtained by the non specific antibodies and the smaller specific antibodies. This was due to the inefficient retention in tumour after the reversal o f the concentration gradient. Consequently, it appears that the functional affinity is important for retention o f specific antibodies within the tumour. Furthermore, the difference between tumour and blood absorbed doses is due to the difference at later timepoints.

Every antibody that was labelled with consistently delivered a larger dose to the tumour

compared to Again this was due to the retention in tumour after clearance fi-om blood

coupled with the longer half-life o f Even the non specific antibodies gave higher tumour

doses when labelled with suggesting that non specific retention also occurred as reported

in other studies (Boxer et al., 1992) where antibody is trapped in the necrotic tumour centre. However, these dose calculations do not account for dose-rate effects which are known to be important.

2.4.2 True dose

This analysis showed that the most important factor was the assumed normal tissue dose limit. This ultimately determined the amount o f activity that could be injected which was

significantly higher for the lower energy compared to Antibodies labelled with

delivered therapy at a higher dose rate but shorter treatment tim e compared to However,

the effective dose to tumour was clearly higher for This supports the results o f other

studies for the use o f longer lived radioisotopes (Howell et al., 1994 & 1998). But, as the injected activity was reduced this became less clear with some antibodies performing better

when labelled with Indeed, this result is likely to be reversed as the injected activity is

further reduced, tumour localisation is impaired and when treating insensitive or highly proliferative tumours (Dale, 1996). Nevertheless, this reinforces the need for accurate characterisation o f absorbed dose, as delivered in RIT, in critical organs.

The analysis also showed larger antibodies and longer lived radionuclides prolonged the effective treatment time. However, increased treatment times also need to be complemented by adequate cumulative radioactivity in tumour and efficient clearance from blood for effective therapy. Consequently, the most effective tumour doses were delivered by specific

antibodies with intermediate clearance rates such as DFM and F(ab’)2. Indeed, one way to

optimise therapy, in both man and animal, is to find a balance between localisation and clearance.

Closely linked to the molecular weight is the valency. There was evidence that increasing valency also improved the effectiveness o f the tumour dose. Indeed, the most successful antibodies in this study were divalent. However, therapeutic enhancement by increasing valency is likely to be limited by longer clearance rates and normal tissue toxicity. This implies that a profitable approach to improving therapy would be to determine the optimal antibody size by varying the valency.

An increase o f four to five orders o f magnitude in antibody affinity improved tumour therapy and also had an effect on treatment time due to improved tumour retention, which supports previous results (Casey et al., 1996; King et al., 1994). However, it should be noted that the affinity measurements were in two distinct and narrow groups, containing the specific and

non-specific antibodies, and the observed effect was more due to specificity than affinity. Nevertheless, antibody affinity is undoubtedly very important and it is likely that an improvement o f affinity would also enhance therapy. However, significant changes in therapeutic efficacy would probably require large improvements in affinity.

The performance o f high affinity antibodies such as A5B7-IgG, A5B7-F(ab’)2 (Lane et al., 1994) and MFE-23 (Begent et al., 1996) has previously been assessed in man. MFE-23 showed similar tumour to blood ratios in man and mouse and had insufficient tumour uptake due to rapid clearance (Verhaar et al., 1995). IgG and F(ab’ ) 2 had a similar rate o f clearance and tumour uptake and there were some responses to treatment in man. By contrast, they behaved very differently in mice. Consequently, more research using multivalent antibodies with intermediate clearance rates is required in man.

There are several potential limitations and sources o f error in the model. One such limitation is the mathematical form for proliferation during treatment. This has been shown to be an important factor (Travis et al., 1987; Dale, 1996) but it is complex and difficult to describe in mathematical terms. Consequently, the form o f proliferation is subject to debate and almost certainly oversimplified in its current form. However, there is little doubt that the effect on therapy o f ongoing proliferation becomes large when dose-rates are small and is therefore difficult to ignore. In addition, proliferation rates may change during treatment. After sterilisation o f sensitive oxygenated cells, the fraction o f hypoxic cells reduces due to their reoxygenation. This causes an apparent acceleration o f the proliferation rate leading to an increase in dose that is used in overcoming proliferation. Again, this may have a

considerable impact when dose-rates are low.

Another source o f error is the lack o f accurate measurements o f the radiosensitivity and rate o f repair for different tumour types. The extent o f each parameter can make a large

difference to therapeutic outcome (Thames et al, 1985; Fowler, 1990). Therefore, it is important to include accurate measurements o f each when assessing the efficacy o f radiolabelled antibodies for RIT.

not the case (Hui et al., 1994). These calculations overestimate the self-absorbed dose in

tumour and normal tissues and the error is likely to be greater for than for In

addition, no account has been taken o f the heterogeneity o f dose deposition and response in tumour. Indeed, the heterogeneity o f radiolabelled antibody in tumour may be a significant factor in the biological effect, particularly if radionuclides with low range emissions are used (Humm, 1990).

Dosimetry calculations were obtained for antibodies linked with and ^^^I and were based

on biodistribution data from antibodies labelled with ^^^I. Although the kinetics o f ^^^I labelled antibodies are likely to be similar to those labelled with ^^^I, this may not be the case

for ^^Y. It is known that will go to bone and hence irradiate bone marrow, if it becomes

detached from the antibody chelate. However, it has previously been shown that the antibodies used in this study have formed stable conjugates with ^ Y without any adverse effects in bone marrow (Casey et al., 1999). In this study, the main differences between antibodies labelled with ^^’l and those labelled with ^^Y occurred at the major sites o f catabolism. There is evidence that radiometals are subject to prolonged intracellular retention and can accumulate in kidney, liver and spleen (Casey et al., 1996). Given the limits imposed by nephrotoxicity, this may have a significant impact when kidney is the main route o f extraction from the circulation. In terms o f biological effect, the impact o f self-dose in liver and spleen is likely to be less pronounced due to the relative radioresistance o f these tissues but there may be significant cross-dose from these organs to bone marrow.

Essentially, the use o f ^^^I biodistribution data for dosimetric evaluations does not take account o f the additional limitations that are specific to ^^Y and, as a result, underestimate the actual dose received by some critical normal tissues (in particular the radiosensitive kidney and bone marrow).

Although the model discussed in this chapter makes some simplifying assumptions, it provides important information about how antibody pharmacokinetics, radionuclide properties, tumour type and dose limiting factors can influence the efficacy o f tumour targeting. This model can readily be used for different tumour types and can be combined with pharmacokinetic models and biodistribution data, in animal systems and in man, to assess the effectiveness. Likewise, it can be used to compare the model values with

measurable biological effects. W ith more accurate measurement o f absorbed dose, it should be possible to obtain a better prediction o f biological outcome from RIT and a more realistic assessment o f therapeutic efficacy.

Contents

Chapter 3:

Validation of techniques for quantifying