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Metastron (Strontium Chloride, SR-89) - Description and Clinical Pharmacology

 
 



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DESCRIPTION

Metastron is a sterile, non-pyrogenic, aqueous solution of Strontium-89 Chloride for intravenous administration. The solution contains no preservative.

Each milliliter contains:

    Strontium Chloride    10.9 - 22.6 mg
    Water for Injection    q.s. to 1 mL

The radioactive concentration is 37 MBq/mL, 1 mCi/mL, and the specific activity is 2.96-6.17 MBq/mg, 80-167 µCi/mg at calibration. The pH of the solution is 4 - 7.5.

PHYSICAL CHARACTERISTICS

Strontium-89 decays by beta emission with a physical half-life of 50.5 days. The maximum beta energy is 1.463 MeV (100%). The maximum range of β- from Strontium-89 in tissue is approximately 8 mm.

Radioactive decay factors to be applied to the stated value for radioactive concentration at calibration, when calculating injection volumes at the time of administration, are given in Table 1.

Table 1: Decay of Strontium-89
Day 1 Factor Day Factor
-24 1.39 +6 0.92
-22 1.35 +8 0.90
-20 1.32 +10 0.87
-18 1.28 +12 0.85
-16 1.25 +14 0.83
-14 1.21 +16 0.80
-12 1.18 +18 0.78
-10 1.15 +20 0.76
-8 1.12 +22 0.74
-6 1.09 +24 0.72
-4 1.06 +26 0.70
-2 1.03 +28 0.68
0=calibration 1.00

1 Days before (-) or after (+) the calibration date stated on the vial.

CLINICAL PHARMACOLOGY

Following intravenous injection, soluble strontium compounds behave like their calcium analogs, clearing rapidly from the blood and selectively localizing in bone mineral. Uptake of strontium by bone occurs preferentially in sites of active osteogenesis; thus primary bone tumors and areas of metastatic involvement (blastic lesions) can accumulate significantly greater concentrations of strontium than surrounding normal bone.

Strontium-89 Chloride is retained in metastatic bone lesions much longer than in normal bone, where turnover is about 14 days. In patients with extensive skeletal metastases, well over half of the injected dose is retained in the bones.

Excretion pathways are two-thirds urinary and one-third fecal in patients with bone metastases. Urinary excretion is higher in people without bone lesions. Urinary excretion is greatest in the first two days following injection.

Strontium-89 is a pure beta emitter and Strontium-89 Chloride selectively irradiates sites of primary and metastatic bone involvement with minimal irradiation of soft tissues distant from the bone lesions. (The maximum range in tissue is 8 mm; maximum energy is 1.463 MeV.) Mean absorbed radiation doses are listed under the Radiation Dosimetry section.

Clinical trials have examined relief of pain in cancer patients who have received therapy for bone metastases (external radiation to indexed sites) but in whom persistent pain recurred. In a multi-center Canadian placebo-controlled trial of 126 patients, pain relief occurred in more patients treated with a single injection of Metastron than in patients treated with an injection of placebo. Results are given in the following tables.

Table 2 compares the percentage and number of patients treated with Metastron or placebo who had reduced pain and no increase in analgesic or radiotherapy re-treatment.

Table 2: Comparison of the effects of Strontium-89 and placebo, as adjunct to radiotherapy, on treatment outcome over time.
Months Post-Treatment
1 2 3 4 5 6
Metastron 71.4% 78.9% 60.6% 59.3% 36.4% 63.6%
(n=42) (n=38) (n=33) (n=27) (n=22) (n=22)
Placebo 61.4% 57.1% 55.9% 25.0% 31.8% 35.0%
(n=44) (n=35) (n=34) (n=24) (n=22) (n=20)

At each visit, treatment success, defined as a reduction in a patient's pain score without any increase in analgesic intake and without any supplementary radiotherapy at the index site, was more frequent among patients assigned to Metastron than to placebo.

Table 3 compares the number and percentage of patients treated with Metastron or placebo as an adjunct to radiotherapy who were pain free without analgesic at the intervals shown.

Table 3: Comparison of the effects of Strontium-89 and placebo, as adjunct to radiotherapy, on reduction of pain score and analgesic score to zero.
Months Post-Treatment
1 2 3 4 5 6 9
Metastron 6 5 5 3 4 4 2
14.3% 13.2% 15.2% 11.1% 18.2% 18.2% 18.2%
(n=42) (n=38) (n=33) (n=27) (n=22) (n=22) (n=11)
Placebo 3 3 2 0 1 1 0
6.8% 8.6% 5.9% 4.5% 5%
(n=44) (n=35) (n=34) (n=24) (n=22) (n=20) (n=17)

The number of patients classified at each visit as treatment successes who were pain free at the index site and required no analgesics was consistently higher in the Metastron group.

New pain sites were less frequent in patients treated with Metastron.

In another clinical trial, pain relief was greater in a group of patients treated with Metastron compared with a group treated with non-radioactive strontium-88.

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