Radioactive drugs ease bone-tumor pain.

Radioactive drugs ease bone-tumor pain

Radiation-emitting chemicals may provide relief for some cancer patients who suffer continuous, toothache-like pain from tumors that have infiltrated their bone. Two of these experimental treatments show impressive success rates and almost no harmful side effects in people whose breast and prostate cancers have spread to bone, researchers reported in St. Louis this week at a meeting of the Society of Nuclear Medicine.

Cancer patients whose malignancies have spread to bone often fail to respond to standard cancer therapies. Physicians usually try to limit tumor growth -- and the pain that accompanies it -- with external radiation, hormones or chemotherapy, but hormones and chemotherapy may provide no pain relief and external radiation is extremely toxic to normal tissue. Its side effects -- which include bone marrow depression, vomiting, diarrhea and lung inflammation -- are especially damaging in patients who harbor many small, dispersed bone tumors, says nuclear medicine physician Harry R. Maxon of the University of Cincinnati Medical Center.

Radioactive cancer drugs cause little or no harm to the patient because they can be given in small, intravenous doses, which the body concentrates near bone tumors, says Ralph G. Robinson at the University of Kansas Medical Center in Kansas City. Once at the tumor site, the drugs give off therapeutic beta-particle radiation that travels only a short distance and so does not affect tissue beyond the bone, he explains.

Robinson's team recently completed trials of radioactive strontium-89--which chemically mimics calcium -- on 28 prostate and four breast cancer patients. They found the injections relieved pain for two to three months in 85 percent of the patients, confirming results of European and Canadian studies and those from Robinson's initial studies in U.S. patients.

"We also found that optimum treatment dose is probably somewhat higher than that used in large numbers of patients in the past. We predict from [our new] results that we will achieve greater than a 90 percent response rate with [this] somewhat higher dose with very acceptable bone marrow toxicity," Robinson says. However, a yet-unpublished British study found no significantly better pain response with higher doses, says nuclear medicine specialist Alexander J. McEwan at the Cross Cancer Institute in Edmonton, Alberta. Strontium-89 has now entered a large-scale patient trial in 30 U.S. hospitals in its last phase before FDA approval, Robinson adds.

In the first human administrations of a radioactive compound called rhenium-186-hydroxyethylidene-diphosphonate, Maxon's team found it alleviated bone pain in 80 percent of 28 prostate and seven breast cancer patients. In addition, injections of the rhenium compound produced "very minimal" bone marrow toxicity and no evident toxicity to any other tissue. "So we hope it will be ready for much more widespread use in a couple of years," Maxon says. The rhenium compound concentrates in metabolically active bone because of its attached phosphonate, which bone absorbs, he explains.

Unlike strontium-89, rhenium-186 gives off imageable gamma-particle emissions in addition to beta particles, enabling scientists to monitor the compound's distribution and amount with ease, Maxon says. Rhenium-186 also has a shorter life in the body than strontium-89 and so theoretically could be given in higher doses, which may be more effective for patients with more advanced disease, McEwan says.

Researchers still don't know how the nuclear therapy works to relieve pain, but they believe it acts to shrink or slow the growth of the tumor at its interface with bone. These drugs do not provide a cure, however. The treatment, notes Robinson, "doesn't totally shrink the tumor to nothing and make it go away."