Pharmacokinetic study of doxycycline in children

Population Pharmacokinetics of Doxycycline in Children

Doxycycline is a tetracycline-class antimicrobial labeled by the United States (U.S.) Food and Drug Administration for children >8 years of age for many common childhood infections. Doxycycline is not labeled for children ≤8 years of age, due to the association between tetracycline class antibiotics and tooth staining, although doxycycline may be used off-label in severe conditions. Accordingly, there is a paucity of pharmacokinetic (PK) data to guide dosing in children 8 years and younger. We leveraged opportunistically-collected plasma samples after intravenous (IV) and oral doxycycline doses received per standard of care to characterize the PK of doxycycline in children of different ages, and evaluated the effect of obesity and fasting status on PK parameters.We developed a population PK model of doxycycline using data collected from 47 patients 0-18 years of age, including 14 participants ≤8 years. We developed a 1 compartment PK model and found doxycycline clearance to be 3.32 L/h/70 kg and volume to be 96.8 L/70kg for all patients; comparable to values reported in adults. We estimated a bioavailability of 89.6%, also consistent with adult data. Allometrically scaled clearance and volume of distribution did not differ between children 2 to ≤8 years of age and children >8 to ≤18 years of age, suggesting that younger children may be given the same per kg dosing. Obese and fasting status were not selected for inclusion in the final model. Additional doxycycline PK samples collected in future studies may be used to improve model performance and maximize its clinical value.

Absorption of doxycycline from a controlled release pellet formulation: the influence of food on bioavailability

A three-way crossover study was performed to compare the bioavailability of a new pelletised doxycycline product administered either with food or without food and a reference product taken without food. Four different methods were used to calculate pharmacokinetic parameters from the data. The sums of squares, Akaike's Information Criterion (AIC), and the ranges for the parameters obtained were used for comparison. Good fits to the data were obtained when all four methods were used, each with a lag time. The two compartment open model was the most efficient method for describing the data. The one compartment open model was the least efficient, particularly with respect to predicting the peak concentration of doxycycline in plasma. All the models gave similar rank order results with respect to bioavailability differences between the three treatments. Analysis of the data by different methods suggests that pelletised doxycycline is bioequivalent to the reference product when taken in the absence of food. A standardized feeding regimen affected the rate, but not extent of absorption of doxycycline from the pelletised formulation.

Pharmacokinetic data on doxycycline and its distribution in different biological fluids in female goats

A pharmacokinetic study of doxycycline after intravenous administration at 5 mg/kg body weight in goats revealed that a concentration of greater than or equal to 0.5 microgram/ml was maintained for 5 min-2 h, 4-12 h, 2-12 h and 5 min-greater than 48 h in plasma, interstitial fluid, milk and urine respectively. The low t1/2 alpha of 0.73 +/- 0.11 h and high t1/2 beta of 16.63 +/- 1.58 h show that the drug is rapidly distributed but slowly eliminated from the body. The tissue:plasma concentration of 4.86 +/- 1.06 during the elimination phase [K12/(K21-beta)] indicates a high expected tissue concentration, which is supported by similarly increased drug concentration in interstitial fluid and milk. The high Vdarea of 9.78 +/- 0.86 L/kg observed denotes that, apart from its wide distribution, the drug may be stored in fat depots as it is known to be highly lipophilic. As the drug maintained a therapeutic concentration for a shorter time in plasma, and the calculated dose rate for maintaining a minimal plasma concentration of 0.5-1.5 micrograms/ml is relatively high, it may not be of much use in treating septicaemia in this species. Since the observed tissue:plasma concentration was higher and a therapeutic concentration was maintained in interstitial fluid and milk for longer, the drug can be used for other systemic infections at a lower dose rate than that required for treating septicaemia. As the drug maintained a very high concentration in urine, it may be of particular value in treating urinary tract infections caused by sensitive micro-organisms.