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CLINICAL PHARMACOLOGY

General

When technetium Tc99m pertechnetate is added to exametazime in the presence of stannous reductant, a lipophilic technetium Tc99m complex is formed. This lipophilic complex is the active moiety. It converts at approximately 12%/hour to less lipophilic species. When the secondary complex is separated from the lipophilic species, it is unable to cross the blood-brainbarrier. The useful life of the reconstituted agent is limited to 30 minutes.

Pharmacokinetics

Studies in normal volunteers have shown that the technetium Tc99m complex of the RR,SS(d,l) diastereoisomer of exametazime is rapidly cleared from the blood after intravenous injection. Uptake in the brain reaches a maximum of 3.5-7.0% of the injected dose within one minute of injection. Up to 15% of the activity is eliminated from the brain by 2 minutes post injection, after which little activity is lost for the following 24 hours except by physical decay of technetium Tc99m. The activity not associated with the brain is widely distributed throughout the body, particularly in muscle and soft tissue. About 30% of the injected dose is found in the gastrointestinal tract immediately after injection and about 50% of this is excreted through the intestinal tract over 48 hours. Also, about 40% of the injected dose is excreted through the kidneys and urine over the 48 hours after injection.

Leukocyte

The lipophilic Tc99m exametazime complex is taken up by leukocytes, and selectively retained in neutrophils. Label elution rate is up to 10% in the first hour.

Pharmacodynamics

Tc99m-labeled leukocyte: Based upon in vivo recovery and very low lung and liver uptake, the labeled leukocytes are still functional. Following reinjection of the Tc99m labeled leukocytes the circulating granulocyte activity as a percentage of labeled granulocyte activity at 40 minutes after injection gave a mean of 37% (range 10-47%).

During the first hour following injection of Tc99m labeled leukocytes, activity is seen in the lungs, liver, spleen, blood pool, bone marrow and the bladder. The kidneys (parenchyma and/or renal pelvis) and gall bladder may also be visualized. Over the first 1-6 hours, the Tc99m is visualized in the bowel. At 24 hours post-injection substantial colonic activity is seen. The normal areas visualized in earlier scans are still visible.

Clinical Trials

Two clinical trials were performed in a total of 88 patients who had suspected intra-abdominal infection or inflammation. Subjects received both Tc99m labeled leukocytes and In-111 labeled leukocytes. Images were obtained at 2 and 30 minutes and at 2 and 4 hours and 24 hours. In two other clinical trials, in a total of 127 patients with suspected abdominal inflammation or infection received Tc99m labeled leukocytes. Imaging was at 24 hours in one study and at 1, 3 and 24 hours in the other. In all four studies images were blindly evaluated and the findings were confirmed by surgery, biopsy or other clinical data.

Based on the above 4 studies, between 2 to 4 hours Tc99m labeled leukocytes had 95-100% sensitivity and 62-85% specificity with similar numbers of false positive and false negative findings. The value of the 24 hour Tc99m labeled leukocyte images is inconsistent. In all studies the false positive and false negatives relate to the bowel background, the location of the site of infection/inflammation and whether or not it is contiguous with the bowel. The 24 hour films should be interpreted with great caution because of a high bowel background; false negatives were noted in both Tc99m and In-111 labeled leukocytes.

Other studies suggest that the interpretation of the images could be affected by the presence of tumors, infarction and peritonitis, etc. Liver abscess may be missed because of the bowel background. Caution should be exercised in making the final diagnosis.

Last reviewed on RxList: 8/14/2013
This monograph has been modified to include the generic and brand name in many instances.

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