Doxycycline is 4-(Dimethylamino)-1,4,4a,5,5a,6,11,12a-Octahydro-3,5,10,12,12a pentahydroxy-6-methyl-1,11-dioxo-2-Naphthacene carboxamide. The molecular formula of doxycycline monohydrate is C22H24N2O8.H2O and a molecular weight of 462.46.Doxycycline is a broad-spectrum semi-synthetic antibiotic, derived from oxytetracycline and is available as doxycycline monohydrate; doxycycline hyclate; doxycycline hydrochloride hemiethanolate hemihydrate and doxycycline calcium for oral administration. Doxycycline has a high degree of lipoid solubility and a low affinity for calcium binding. It is highly stable in normal human serum. It acts bacteriostatically by binding with ribosome and thus inhibiting the bacterial protein synthesis. It is used against organisms like Escherichia coli, Mycoplasma pneumoniae, Streptococcus pneumoniae, Haemophilus influenzae and Enterococcus faecalis. It is considered the drug of choice for atypical pneumonia, anthrax, rickettsial diseases, cholera, urinary tract infection and for sexually transmitted diseases (Natalie and Cunha, 1995).The drug is mainly eliminated through the intestinal tract and in the urine (about 40% in 72 h).
Therefore studies of mechanisms involved in the renal handling are of practical
importance. The rate of glomerular filtration can be determined by measuring
the renal clearance of substances that are neither secreted nor reabsorbed by
the tubules and exert no effect on renal function. In all mammalian species,
creatinine is freely filtrated in the same concentration as in plasma, so renal
clearance of creatinine is a widely used parameter for the assessment of kidney
function (Bowers and Wong, 1980). The creatinine clearance value is determined
from the creatinine concentration in serum and pooled 24 h urine samples. A
direct measure would be the renal clearance of creatinine measured during each
period of urine collection. As creatinine is predominantly excreted via glomerular
filtration, so regarded as direct measure of glomerular filtration. The present
research project was designed to study the renal clearance of creatinine and
doxycycline in male volunteers under local environmental conditions. As the
biochemical parameters recorded under indigenous conditions differs from that
of foreign counterpart due to Geometric difference (Nawaz and Shah, 1985; Nawaz
et al., 1984; Nawaz, 1994).
Materials and Methods
Drug and administration and sampling
Renal clearance of endogenous creatinine and doxycycline was determined
in 8 healthy adult male volunteers following 2x100 mg tablets of doxycycline.
The physical parameters include average body weight, age and heights of volunteers
were noted. The blood samples were drawn in heparinized centrifuge tubes at
0, 30, 60, 90, 120, 150, 180 and 240 min after the drug administration. Urine
samples were taken at 0, 30, 60, 90, 120, 150 and 180 min. The blood samples
were centrifuged at 4,000 rpm for 15 min to separate plasma and stored at -20°C
for further analysis.
For the estimation of glomerular filtration rate (GFR), the endogenous creatinine
renal clearance was measured in plasma and urine samples spectrophotometrically
by the method of Bonsnes and Taussky (1945).To 0.5 ml of plasma, 4 ml of N/12
H2SO4 and 0.5 ml of 10% sodium wulfamate was added and
centrifuged at 4000 rmp for 15 min. To 3 ml of supernatant, 2 ml of fresh alkaline
picrate were added and absorbance was noted at 515 nm wavelength. First for
samples of urine were diluted to 1:50 and in 3 ml of each dilution, 2 ml of
freshly prepared alkaline picrate was added and absorbance was noted.
Doxycycline concentration in plasma and urine was determined by microbiological
assay according to the method of Arret et al. (1971) by using streptococcus
fecalus as test organism. Paper disks were placed on surface of medium containing
nutrient agar and incubate at 37°C for 18 hours and the zones of inhibition
were measured. Different parameters were calculated as:
Results and Discussion
The results showing mean ± SE of the experimental periods of pH of blood and urine, diuresis, plasma and urine concentration of creatinine, doxycycline concentration, doxycycline clearance and clearance ratio have been presented in Table 1 and graphically shown in different figures.
In this study the average ± SE value of pH in blood was 7.81 ±
0.03. This value is almost equal to the previously reported value of blood pH
7.73 ± 0.02 in male volunteers. Average±SE value of pH in urine
was 6.13±0.187 which is comparable to the value 5.85 ± 0.08 reported
earlier on male volunteers. The average±SE value for the rate of urine
flow (diuresis) recorded in human male volunteers was 0.056±0.014 ml/min/kg.
This value is higher than that reported earlier in winter season which was 0.04±0.004
ml/min/kg (Naseem, 1999).
||Average data of renal clearance of endogenous creatinine and
doxycycline in male volunteers after oral dose of 100x2 mg (200) tablets
This difference is due to environmental difference because in hot climate the
evaporation reduces the urine flow during summer while lower environmental temperature
increases the rate of urine during winter (Nawaz and Shah, 1984).
In the present study the average ± SE value of creatinine clearance in male volunteers was 1.109±0.297 ml/min/kg. It slightly differ than the values reported earlier for male volunteers i.e. 1.95±0.164 ml/min/kg (Yasin et al., 2001) and for female volunteers i.e. 1.13±0.07 ml (min/ml/kg) (Naseem, 1999).
The average ± SE value for the doxycycline concentration in plasma was 2.89 ± 0.166 μg/ml and the results were comparable to earlier study of (Taneja et al., 1974) in which the plasma drug concentration of earlier study in almost similar, so age and gender has no effect on bio-disposition of drug. That is why dose adjustment based on age and gender is not necessary as the peak serum concentration reached at the same time in all subjects after dosing (Saux et al., 1982).
The average±SE value for the renal clearance of doxycycline was 0.491±0.056 ml/min/kg. The present value is slightly lower than the renal clearance value 0.538 ml/min/kg body weight reported by (Taneja et al., 1974) due to difference of environment and temperature.
Average±SE value for renal clearance ratio was 1.046±0.35. From the data it is cleared that clearance of doxycycline was lower than the endogenous creatinine clearance or filtration clearance (GFR). This indicates that re-absorption or back diffusion take place (Taneja et al., 1974).
The studies indicate that the renal clearance of endogenous creatinine in male volunteers was lower than the values given in the literature. There is significant positive correlation between diuresis and doxycycline clearance (Fig. 1). As the rate of urine flow increases, rate of doxycycline clearance also increases. There is slightly significant positive correlation between pH of urine and doxycycline clearance (Fig. 2).This is because in acidic pH of urine drug becomes unionized and is reabsorbed and with increase in pH it becomes ionized hence renal clearance increases.
|| Relationship between diuresis and renal clearance of drug
|| Relationship between pH of urine and renal clearance of drug
|| Relationship between plasma concentration of drug and renal
clearance of drug
The non-significant positive correlation between doxycycline and plasma concentration and renal clearance (Fig. 3) shows that when drug is retained in plasma, renal clearance decreases and vice versa. The lower values of renal clearance support the previous findings that there is a need to evaluate the imported drug under indigenous conditions for getting optimal therapeutic effects and to highlight the lacunae in our knowledge of the drugs pharmacodynamics in human body.