Glofil-125 (Iothalamate Sodium, I-125) - Description and Clinical Pharmacology

DESCRIPTION

General

GLOFIL^®-125 (Sodium Iothalamate I-125 Injection) is a sterile, nonpyrogenic aqueous injection containing approximately 1 mg sodium iothalamate per mL, and 0.9 percent benzyl alcohol as a preservative. The radioactive concentration of the material is 250-300 μCi/mL as of the calibration date. Sodium bicarbonate and hydrochloric acid are present for pH adjustment.

Physical Characteristics

Iodine-125 decays by electron capture with a physical half-life of 60.14 days. Photons that are useful for detection are listed in Table 1.

Table 1. Principal Radiation Emission Data ¹

Radiation	Mean Number Per Disintegration	Mean Energy (keV)
Gamma-1	0.067	35.5
Kα1 X-ray	0.741	27.5
Kα2 X-ray	0.398	27.2
Kß1 X-ray	0.140	31.0
Kß2 X-ray	0.043	31.7
Kß3 X-ray	0.072	30.9

The specific gamma ray constant for I-125 is 1.43 R/mCi-hr at 1 cm. The first half value thickness of lead (Pb) for I-125 is 0.017 mm. A range of values for the relative attenuation of the radiation emitted by this radionuclide resulting from interposition of various thicknesses of Pb is shown in Table 2. For example, the use of 0.28 mm of Pb will decrease the external radiation exposure by a factor of 10,000.

Table 2. Radiation Attenuation by Lead Shielding ¹

Shield Thickness (Pb), mm	Coefficient of Attenuation
0.017	0.5
0.058	0.1
0.12	0.01
0.2	0.001
0.28	0.0001

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

Table 3. Physical Decay Chart; I-125, half-life 60.14 days

Days	Fraction Remaining	Days	Fraction Remaining	Days	Fraction Remaining
0 1	1.000	15	0.841	30	0.707
1	0.989	16	0.831	31	0.699
2	0.977	17	0.822	32	0.691
3	0.966	18	0.812	33	0.683
4	0.955	19	0.803	34	0.675
5	0.944	20	0.794	35	0.667
6	0.933	21	0.785	36	0.660
7	0.922	22	0.776	37	0.652
8	0.912	23	0.767	38	0.645
9	0.901	24	0.758	39	0.637
10	0.891	25	0.749	40	0.630
11	0.881	26	0.740	41	0.623
12	0.871	27	0.732	42	0.616
13	0.861	28	0.724	43	0.608
14	0.851	29	0.715	44	0.601
				45	0.595

CLINICAL PHARMACOLOGY

The renal clearance of sodium iothalamate in man closely approximates that of inulin. The compound is cleared by glomerular filtration without tubular secretion or reabsorption. Following infusion administration of I-125 iothalamate, the effective half-life is about 0.07 days.