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Preclinical studies of [99mTc]DTPA-mannosyl-dextran☆

https://doi.org/10.1016/S0969-8051(03)00028-3Get rights and content

Abstract

We report the preclinical testing of a synthetic receptor-binding macromolecule, [99mTc]DTPA-mannosyl-dextran (36 kDa, 8 DTPA and 55 mannosyl units per dextran, KD = 0.12 nM), for sentinel node detection. Nonclinical safety studies included cardiac pharmacology safety studies, acute toxicology and pathology studies at 50 and 500 times the scaled human dose in both rats and rabbits after foot pad administration, and perivascular irritation studies in rabbits following intra-muscular administration at 100 and 1000 times the scaled human dose. Biodistribution studies in rabbits at 15 m, 1 h, and 3 h indicated that [99mTc]DTPA-mannosyl-dextran cleared the hind foot pad with a biological half-life of 2.21 ± 0.27 h. Other than mild hepatocyte hypertrophy in rabbits, no abnormalities in toxicology or pathology were found. Intravenous administration had no effect on survival, any clinical observations, electrocardiograms, or blood pressures. Intramuscular injection had no effect on survival, clinical observations, injection site observations, or injection site histopathology. The estimated absorbed radiation dose to the affected breast was 0.15 mGy/MBq and the effective dose was 1.06 x 10-2 mSv/MBq. This preclinical study demonstrates that [99mTc]DTPA-mannosyl-dextran has no toxicities and has an acceptable biodistribution and radiation dose.

Introduction

The ideal properties of a radiotracer for sentinel node lymphoscintigraphy and gamma probe detection in breast cancer include: rapid clearance of the injection site, high uptake in the sentinel node, safe radiation exposure levels in the patient and health care personnel, and a lack of local or systemic toxicity in the patient. [99mTc]DTPA-mannosyl-dextran [24], is a new synthetic macromolecule specifically designed for sentinel node lymphoscintigraphy and gamma probe detection. This radiopharmaceutical accumulates in lymphatic tissue by binding to a receptor that resides on the surface of macrophages in the lymph node [8], [9]. The application of sentinel node imaging and gamma probe detection has become more routine over the last few years [10], [14]. A radiopharmaceutical with improved biologic properties such as rapid injection site clearance and persistent sentinel node uptake may be important in resolving the sentinel node focus from the intense breast injection site especially if the injection site is in the upper outer quadrant of the breast.

Preclinical studies performed on this new radiotracer included biodistribution in rabbits, toxicology-pathology in rats and rabbits, cardiac pharmacology safety in dogs, and perivascular irritation in rabbits. The absorbed radiation dosimetry for humans was calculated based on the rabbit biodistribution studies and the MIRD method.

Section snippets

Precursor synthesis

Diethylenetriamine pentaacetic acid mannosyl-dextran (DTPA-mannosyl-dextran) was prepared as previously described [24], using pharmaceutical grade dextran (PM-10, Amersham-Pharmacia Biotech, Piscataway, NJ).

Radiolabeling system

In preparation for a Phase I clinical trial we developed a radiolabeling system that employed a two-component kit: a “Labeling Vial” containing the drug precursor, DTPA-mannosyl-dextran, and a “Diluent Vial” containing phosphate buffered saline.

Twenty “Labeling Vials” were prepared using

Radiopharmaceutical preparation

The [99mTc]DTPA-mannosyl-dextran preparation used throughout this study consisted of 8 DTPA, 23 amino groups, and 55 mannosyl units per dextran, and had a molecular weight of 35,800 grams/mole and a molecular diameter of 7.1 ± 0.9 nm. Typical labeling yields were in excess of 98% and were maintained over six months of storage.

Biodistribution studies

Table 1 lists the mean values ± the coefficient-of-variation of percent of injected dose per organ and the percent-of-injected dose per gram of each tissue sampled at 5 m,

Discussion

[99mTc]DTPA-mannosyl-dextran is the first in a class of diagnostic agents designed for targeted delivery of MRI and nuclear imaging reporters. For this reason, successful preclinical testing of this agent opens the possibility of targeting other tissues by replacing the mannosyl units with other receptor substrates. The following discusses some of the considerations that preceded the preclinical assessment this agent and an evaluation of the results.

Pharmaceutical (PM) grade dextran, available

Conclusion

This preclinical study demonstrated that [99mTc]DTPA-mannosyl-dextran has no toxicities, an acceptable biodistribution, and a low absorbed radiation dose.

Acknowledgements

This work was supported in part by National Cancer Institute Grant CA72751 and University of California Breast Cancer Research Program Grants 2RB-0018 and 4IB-0051. We thank Neoprobe Corporation for the use of a Model 1000 portable radioisotope detector and Amersham-Pharmacia Biotech for their generous gift of PM-10 dextran.

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    Supported in part by National Cancer Institute Grant CA72751 and University of California Breast Cancer Research Program Grants 2RB-0018 and 4IB-0051.

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