AN-Dtpa (Technetium Tc 99m Pentetate Calcium Trisodium) - Description and Clinical Pharmacology

DESCRIPTION

AN-DTPA^® Kit for the Preparation of Technetium Tc 99m Pentetate Injection is a multidose reaction vial which contains the sterile, non-pyrogenic, non-radioactive ingredients necessary to produce Technetium Tc 99m Pentetate Injection for diagnostic use by intravenous injection.

Each 10 mL reaction vial contains 20.6 mg pentetate calcium trisodium, 0.15 mg minimum stannous tin as stannous chloride dihydrate and 0.30 mg maximum total tin as stannous chloride dihydrate in lyophilized form and sealed under nitrogen. The pH is adjusted to 3.9-4.1 with sodium hydroxide and/or hydrochloric acid prior to lyophilization. No bacteriostatic preservative is present.

The active agent is a Technetium Tc 99m complex of pentetate calcium trisodium. The chemical names and structure for pentetate calcium trisodium are: (1) Calciate (3-), [N,N-bis[2-[bis(carboxymethyl)amino]ethyl]glycinato(5-)]-,trisodium;(2)Trisodium [N,N-bis[2-[bis(carboxymethyl)amino]ethyl]glycinato(5-)]-calciate(3-).

When a solution of sterile, non-pyrogenic, oxidant-free isotonic Sodium Pertechnetate Tc 99m Injection is added to the vial, Technetium Tc 99m Pentetate Injection is formed. The product so derived is intended for intravenous injection within 6 hours of the time of reconstitution. The precise structure of Technetium Tc 99m Pentetate Injection if not known at this time.

Physical Characteristics

Technetium Tc 99m decays by isomeric transition with a physical half-life of 6.02 hours.¹ The principal photon that is useful for detection and imaging studies is listed in Table 1.

Table 1. Principal Radiation Emission Data

Radiation	Mean Percent Per Disintegration	Mean Energy (keV)
Gamma-2	89.07	140.5

 ¹Kocher DC: Radioactive decay data tables . DOE/TIC-11026:  108, 1981

External Radiation

The specific gamma ray constant for Tc 99m is 0.78 R/millicurie-hr at 1 cm. The first half-value layer is 0.017 cm of lead (Pb). A range of values for the relative attenuation of the radiation emitted by this radionuclide that results from interposition of various thicknesses of Pb is shown in Table 2. For example, the use of a 0.25 cm thickness of Pb will attenuate the radiation emitted by a factor of about 1,000.

Table 2. Radiation Attenuation by Lead Shielding

Shield Thickness (Pb) cm	Coefficient of Attenuation
0.017	0.5
0.08	10-1
0.16	10-2
0.25	10-3
0.33	10-4

To correct for physical decay of this radionuclide, the fractions that remain at selected intervals after the time of calibration are shown in Table 3.

Table 3. Physical Decay Chart: Tc 99m, half-life 6.02 hours

Hours	Fraction Remaining	Hours	Fraction Remaining
0*	1.000	7	0.447
1	0.891	8	0.398
2	0.794	9	0.355
3	0.708	10	0.316
4	0.631	11	0.282
5	0.562	12	0.251
6	0.501

*Calibration time

CLINICAL PHARMACOLOGY

Following its intravenous administration. Technetium Tc 99m Pentetate Injection rapidly distributes itself throughout the extracellular fluid space from which it is promptly cleared from the body by glomerular filtration. There should be little or no binding of the chelate by the renal parenchyma. A variable percentage of the Technetium Tc 99m Pentetate binds to the serum proteins; this ranges from 3.7% following a single injection to approximately 10% if the material is continuously infused. Although the chelate gives useful information on the glomerular filtration rate, the variable percent which is protein bound leads to a measured clearance rate which is lower than that determined by inulin clearance.

The images of the kidneys obtained in the first few minutes after administration of Technetium Tc 99m Pentetate Injection represent the vascular pool within the kidney. Subsequent images of the kidneys represent radioactivity which is in the urine of both the collecting system and the renal pelvis.

Technetium Tc 99m Pentetate Injection tends to accumulate in intracranial lesions with excessive neovascularity or an altered blood-brain barrier. It does not accumulate in the choroid plexus.