Assessment of radioactive residues arising from radiolabel instability in a multiple dose tissue distribution study in rats.
Our study objectives were: To quantitatively determine the effect of radiolabel instability on terminal phase radioactive tissue residues in a multiple dose tissue distribution study. To quantitatively compare tissue residue artifacts (non drug-related radioactivity) from two chemically-distinct radiolabel locations. To conduct a definitive multiple dose tissue distribution study using the better of the two radiolabeled compounds. We compared the excretion and tissue distribution in rats of [(14)C]linezolid, radiolabeled in two different locations, after 7 consecutive once daily [(14)C] oral doses. The radiolabels were in the acetamide (two carbon) and oxazolidinone (isolated carbon) functional groups. Terminal phase tissue residue and excretion data were compared to data from rats dosed orally with [(14)C]sodium acetate. Drug-related radioactivity was excreted rapidly over 24 h. After a single dose, the acetamide and oxazolidinone radiolabel sites both gave 3% of dose as exhaled (14)CO(2). After 7 daily [(14)C] oral doses, terminal phase radioactive tissue residues were higher from the acetamide radiolabel, relative to the oxazolidinone radiolabel, and were primarily not drug-related. In the definitive tissue distribution study, low concentrations of drug-related radioactivity in skin and thyroid were observed. We conclude that although small amounts of radiolabel instability do not significantly affect single dose tissue radioactivity C(max) and AUC, artifacts arising from radiolabel instability can prolong the apparent terminal phase half life and complicate study data interpretation. When possible, it is always preferable to use a completely stable radiolabel site.
Product Life Cycle Management, Pharmacia Corp., Peapack, NJ 07977, USA. j.greg.slatter@pharmacia.com
John Gregory
JG
Slatter
James Peter
JP
Sams
John Alan
JA
Easter
Gwendolyn Deane
GD
Fate
Koji
K
Chiba
Mark Gerard
MG
Johnson
Maria
M
Courtney
Michael Dennis
MD
Koets
Lorrie Renee
LR
Norris
Brian Wayne
BW
Jones
D000081:Acetamides / Q000008:administration & dosage / Q000493:pharmacokinetics*; D000284:Administration, Oral; D000818:Animals; D000900:Anti-Bacterial Agents / Q000008:administration & dosage / Q000493:pharmacokinetics*; D019540:Area Under Curve; D002250:Carbon Radioisotopes / Q000493:pharmacokinetics*; D004355:Drug Stability; D006207:Half-Life; D000069349:Linezolid; D008297:Male; D008657:Metabolic Clearance Rate; D023303:Oxazolidinones / Q000008:administration & dosage / Q000493:pharmacokinetics*; D051381:Rats; D017207:Rats, Sprague-Dawley; D019346:Sodium Acetate / Q000493:pharmacokinetics; D013997:Time Factors; D014018:Tissue Distribution
D000081:Acetamides; D000900:Anti-Bacterial Agents; D002250:Carbon Radioisotopes; D023303:Oxazolidinones; D019346:Sodium Acetate; D000069349:Linezolid