Tc-99(m)-citrate: A new bone imaging radiopharmaceutical

Tc-99(m)-citrate has been shown to incorporate irreversibly in the skeleton with a biodistribution different to that on a conventional bone scan. The aims of this study were to confirm the bone-imaging properties of Tc-99(m)- citrate, to identify the variabilities influencing its skeletal uptake, to compare its uptake in bone with that of Tc-99(m)-MDP and to assess its poss ible role in bone scintigraphy. Appropriate animal and human studies (n = 4 5) were conducted. The 3 h lesion-to-bone ratio of Tc-99(m)-citrate was com pared with that of Tc-99(m)-MDP in more than 150 lesions, including osteobl astic sites (Group A), lesions undergoing treatment or healing (Group B), a nd degenerative or old healed lesions (Group C). The uptake ratio was class ified as concordant (< 20% variation), mildly discordant (20-50% variation) or significantly discordant (> 50% variation). Animal experiments showed m ost bone uptake of Tc-99(m)-citrate when prepared at a pH of 6.0-6.5. The t wo radiopharmaceuticals appeared to compete for bone uptake, suggesting rel ated but different sites of bone accumulation. The Tc-99(m)-citrate/Tc-99(m )-MDP uptake ratio in Group A was concordant (mean +/- S.D. = 0.92 +/- 0.15 ), while Group C lesions had a significantly lower Tc-99(m)-citrate/Tc-99(m )-MDP uptake ratio (0.34 +/- 0.24, P < 0.01). A comparison of Group B lesio ns showed wide variation in intensity and area of involvement in many lesio ns (uptake ratio <0.5 or >1.5 in 13 of 30 sites). We conclude that Tc-99(m) -citrate has a different site of bone localization than phosphonates, possi bly in the organic matrix. Although its skeletal uptake is lower than that of Tc-99(m)-MDP, it may have better specificity in differentiating osteobla stic from degenerated or healed bony lesions, and therefore be useful in pr edicting healing of bone secondaries, fractures or osteomyelitic lesions. ( (C) 1999 Lippincott Williams & Wilkins).