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Deep vein thrombosis (DVT) and pulmonary embolism (PE) are different aspects of a single condition, venous thrombo-embolic disease (VTE), a major cause of morbidity and mortality in the western world. Rapid diagnosis is critical, as timely medical intervention can have a substantial beneficial effect on the mortality rate.

Irrespective of its presentation, VTE is a difficult disease to diagnose. Pathologies unrelated to VTE can give rise to a clinical presentation similar to DVT or PE, resulting in a false positive diagnosis. This raises the risk of a patient being treated inappropriately. Therefore, there is a need for an agent that has high specificity and sensitivity for the detection of active blood clots, which are amenable to treatment by anticoagulant and/or thrombolytic therapy.

This work describes the pre-clinical efficacy studies performed on one such agent, ^99mTc-NC100668. ^99mTc-NC100668 is a substrate for factor XIIIa and as a potential physiological, rather than anatomical, marker of VTE it is hoped it will not give rise to the false negative and positive diagnoses that are inherent in the currently available diagnostic techniques, such as the ventilation perfusion (V/Q) scan, multidetector computer tomography or ultrasound.

It is reported in this work that ^99mTc-NC100668 uptake and retention in blood clot was rapid and maintained over at least a 4 hour period in a rat model of DVT. Anticoagulant and thrombolytic therapies commonly used to treat thrombosis did not seriously impair the ability of ^99mTc-NC100668 to detect thrombi. No significant tissue retention, which could interfere with the ability to image thrombi in vivo, was observed. Biodistribution and plasma clot uptake studies showed that ^99mTc complex of gly-NC100194, the major metabolite of ^99mTc-NC100668, would be unlikely to affect adversely the clinical utility of the test substance.

The in vitro uptake of ^99mTc-NC100668 into forming plasma clots indicated that retention into human blood clots would be comparable with the observations made in the rat preclinical models.

The uptake of ^99mTc-NC100668 in vitro and in vivo was much greater than could be accounted for by physical entrapment into the forming blood clots. The reduced uptake of a biologically inactive analogue of ^99mTc-NC100668 both in vitro and in vivo indicated that the blood clot uptake and retention of ^99mTc-NC100668 was mediated by factor XIIIa.

In conclusion, ^99mTc-NC100668 might be useful in the detection of thrombo embolism.