Improving ion-trap GC-MS quantitation limits for therapeutic agents extracted from rat plasma.

Insufficient quantitation limits using ion-trap gas-chromatography mass-spectrometry (GC-MS) prevented the assay of some samples during a preliminary screening of preclinical rat plasma samples (50 microliter) containing novel, polar therapeutic agents. Few options were available for improving the lower limit of quantitation. The limited amount of sample available precluded the extraction additional plasma. Lipid-liquid extraction recoveries were greater than 90% throughout the range of the standard curve (500-2000 ng ml-1). Chromatography was optimized and multiple, equivalent sites for analyte fragmentation were precluded, using MS-MS to improve assay sensitivity. Quantitation limits were decreased 10-fold however, by using a larger syringe to increase the injection volume from 5 to 50 microliter, in combination with a universal programmable injector. These large injection volumes required changes in the injector events program and in column plumbing. Additionally, evaluation of injection liner packing material demonstrated a 2-fold improvement in sensitivity, using carbofrit, relative to silanized glass wool. Converting to inert ion-trap electrodes did not appear to affect the detection limit, perhaps due to over-riding peak broadening during gas chromatography. The changes described produced a 20-fold improvement in the lower limit quantitation.

Changes in renal function in cats following treatment of hyperthyroidism using 131I.

Changes in renal function of twenty-two cats treated for hyperthyroidism using radioiodine were evaluated. Serum thyroxine (T4), serum creatinine, blood urea nitrogen (BUN) and urine specific gravity were measured before treatment and 6 and 30 days after treatment. Twenty-two cats had pretreatment and 21 cats had 6 day posttreatment measurement of glomerular filtration rate (GFR) using nuclear medicine imaging techniques. There were significant declines in serum T4 at 6 days following treatment, but the changes in GFR, serum creatinine and BUN were not significant. At 30 days following treatment, there were significant increases in BUN and serum creatinine and further significant declines in serum T4. Nine cats were in renal failure prior to treatment and 13 cats were in renal failure 30 days following treatment. Renal failure was defined as BUN greater than 30 mg/dl and/or serum creatinine greater than 1.8 mg/dl with concurrent urine specific gravity less than 1.035. These 13 cats included eight of 9 cats in renal failure prior to treatment and 5 cats not previously in renal failure. Follow up information beyond 30 days following treatment on 9 of these 13 cats indicated that all remained in renal failure. Based on receiver operating curve analysis of pretreatment glomerular filtration rate (GFR) in predicting posttreatment renal failure, a value of 2.25 ml/kg/min as a point of maximum sensitivity (100%) and specificity (78%) was derived. Fifteen of 22 cats had pretreatment GFR measurements of less than 2.25 ml/kg/min. These 15 cats included all 9 cats in renal failure and 5 cats with normal renal clinicopathologic values prior to treatment. At 30 days following treatment, 13 of these 15 cats were in renal failure. The 2 cats not in renal failure had persistently increased serum T4 values. Seven of 22 cats had pretreatment GFR measurements greater than 2.25 ml/kg/min. None of these 7 cats was in renal failure at 30 days following treatment, all cats having normal BUN, serum creatinine, and urine specific gravity values. It was concluded that significant declines in renal function occur after treatment of hyperthyroidism and this decline is clinically important in cats with renal disease. Pretreatment measurement of GFR is valuable in detecting subclinical renal disease and in predicting which cats may have clinically important declines in renal function following treatment.