Rules of thumb for maximum percent tumor accumulation

A Revisit to the Pretargeting Concept-A Target Conversion

Pretargeting is often used as a tumor targeting strategy that provides much higher tumor to non-tumor ratios than direct-targeting using radiolabeled antibody. Due to the multiple injections, pretargeting is investigated less than direct targeting, but the high T/NT ratios have rendered it more useful for therapy. While the progress in using this strategy for tumor therapy has been regularly reviewed in the literature, this review focuses on the nature and quantitative understanding of the pretargeting concept. By doing so, it is the goal of this review to accelerate pretargeting development and translation to the clinic and to prepare the researchers who are not familiar with the pretargeting concept but are interested in applying it. The quantitative understanding is presented in a way understandable to the average researchers in the areas of drug development and clinical translation who have the basic concept of calculus and general chemistry.

Use of Morpholino Oligomers for Pretargeting

Differing from the conventional direct-targeting strategy in which a probe or payload is directly loaded onto a targeting molecule that binds to the native target, pretargeting is an improved targeting strategy. It converts the native target to an artificial target specific for a secondary targeting molecule loaded with the probe or payload (effector). The effector is small and does not accumulate in normal tissues, which accelerates the targeting process and generates high target to nontarget ratios. DNA/cDNA analogs can serve as the recognition pair, i.e., the artificial target and the secondary targeting effector. Morpholino oligomers are so far the most investigated and the most successful DNA/cDNA analog recognition pairs for pretargeting. Herein, we describe the pretargeting principles, the pretargeting strategy using Morpholino oligomers, and the preclinical success so far achieved.

Comparison between two labeled agents in mice using a coinjection-ratio approach in contrast to a conventional group approach

INTRODUCTION

The differences between two agents often need to be accurately defined in vivo. Usually they are injected respectively into two groups of subjects. However, if the two agents do not interact with each other in vivo, a coinjection would serve the same purpose. We believe some individual differences in biodistribution may be circumvented through this approach by calculating organ level ratios.

METHODS

A model system of MORF/cMORF pretargeting (MORF/cMORF is a complementary pair of DNA analogues) was employed in connection with an on-going tumor therapeutic project. Human LS174T cells were implanted into the flank of severely immuno-compromised NOD-scid IL2rg(null) mice. The tumor was confirmed to express TAG-72 antigens. At 16 days post tumor inoculation, mice received IV 60 μg of MORF-conjugated CC49 (an antiTAG-72 antibody), followed 2 days later by a low-mass-dose IV coinjection containing 2.5 μg of (90)Y-cMORF and 2.5 μg of (99m)Tc-cMORF. At 3 h post radioactivity injection, the distribution of (99m)Tc was imaged on a SPECT/CT camera and then organs were excised and counted for (90)Y and (99m)Tc. Because the two labeled cMORFs do not react or interact with each other in vivo, the two groups of (90)Y and (99m)Tc data enabled a conventional group comparison. In a new effort, (90)Y/(99m)Tc ratios were calculated. Student's t-test and retrospective power analysis were performed for both approaches. In the new approach, the ratios were set at 1 as the null hypothesis.

RESULTS

The Student's t-test in the conventional group approach indicated that the two labeled cMORFs distributed similarly, but significant differences were observed in salivary gland and large intestines. The coinjection-ratio approach certainly did not subvert the results of the conventional approach but revealed subtler differences. The P values were reduced, the powers were increased in most organs, and more significant differences were observed. The increased sensitivity was due to the reduced CV%s (SD/average*100%) of the (90)Y/(99m)Tc ratios. Therefore, some individual differences were circumvented and notably the ratio approach differentiated individual differences into ratio-correctable and ratio-uncorrectable.

CONCLUSIONS

Although the conventional approach is reliable, the coinjection-ratio approach using organ level ratios is more sensitive and therefore is recommended whenever possible. In addition, it differentiates individual differences into "coinjection correctable" and "coinjection uncorrectable".