e of calibration), respectively. The formulation for the dosimetric and the therapeutic dosage forms contains 4.4%−6.6% (w/v) povidone, 1−2 mg/mL maltose (dosimetric dose) or 9−15 mg/mL maltose (therapeutic dose), 8.5−9.5 mg/mL sodium chloride, and 0.9−1.3 mg/mL ascorbic acid. The pH is approximately 7.0.
BEXXAR Therapeutic Regimen
The BEXXAR therapeutic regimen is administered in two discrete steps: the dosimetric and therapeutic steps. Each step consists of a sequential infusion of Tositumomab followed by Iodine I 131 Tositumomab. The therapeutic step is administered 7-14 days after the dosimetric step. The BEXXAR therapeutic regimen is supplied in two distinct package configurations as follows:
BEXXAR Dosimetric Packaging
A carton containing two single-use 225 mg vials and one single-use 35 mg vial of Tositumomab supplied by McKesson BioServices and
A package containing a single-use vial of Iodine I 131 Tositumomab (0.61 mCi/mL at calibration), supplied by MDS Nordion.
BEXXAR Therapeutic Packaging
A carton containing two single-use 225 mg vials and one single-use 35 mg vial of Tositumomab, supplied by McKesson BioServices and
A package containing one or two single-use vials of Iodine I 131 Tositumomab (5.6 mCi/mL at calibration), supplied by MDS Nordion.
Physical/Radiochemical Characteristics of Iodine-131
Iodine-131 decays with beta and gamma emissions with a physical half-life of 8.04 days. The principal beta emission has a mean energy of 191.6 keV and the principal gamma emission has an energy of 364.5 keV (Ref 1).
External Radiation
The specific gamma ray constant for Iodine-131 is 2.2 R/millicurie hour at 1 cm. The first half-value layer is 0.24 cm lead (Pb) shielding. A range of values is shown in Table 1 for the relative attenuation of the radiation emitted by this radionuclide that results from interposition of various thicknesses of Pb. To facilitate control of the radiation exposure from this radionuclide, the use of a 2.55 cm thickness of Pb will attenuate the radiation emitted by a factor of about 1,000.
Table 1: Radiation Attenuation by Lead Shielding Shield Thickness (Pb) cm
Attenuation Factor
0.24
0.5
0.89
10-1
1.60
10-2
2.55
10-3
3.7
10-4
The fraction of Iodine-131 radioactivity that remains in the vial after the date of calibration is calculated as follows:
Fraction of remaining radioactivity of Iodine-131 after x days = 2-(x/8.04).
Physical decay is presented in Table 2.
Table 2: Physical Decay Chart: Iodine-131: Half-Life 8.04 Days Days
Fraction Remaining
0*
1.000
1
0.917
2
0.842
3
0.772
4
0.708
5
0.650
6
0.596
7
0.547
8
0.502
9
0.460
10
0.422
11
0.387
12
0.355
13
0.326
14
0.299
*(Calibration day)
CLINICAL PHARMACOLOGY
General Pharmacology
Tositumomab binds specifically to the CD20 (human B-lymphocyte−restricted differentiation antigen, Bp 35 or B1) antigen. Thi
