In neonates, AOP cast serious clinical concerns about the negative effects of dysoxia [1], secondary oxidative stress damage with lipid peroxidation and subsequent neurological problems [12]. Caffeine for AOP treatment is the most widely used drug in neonatology. Its use is associated to reduction of bronchopulmonary dysplasia [13, 14], and improvement of long-term neurodevelopmental outcome [15–17].
The wide caffeine safety interval restricts the caffeine level determinations, performing them just when there is a suspicion of intoxication or a poor therapeutic response with an adequate dosing. Nevertheless, routinely neonatologists adjust caffeine dose in AOP according to the expected response raising it even above the recommended doses to avoid mechanical ventilation, or lowering it when a suspect side effects appear [8].
Our study shows a high correlation between saliva and serum caffeine concentrations in premature infants. Pharmacokinetics and pharmacodynamic studies are necessary in neonates where available data are scarce in most of the drugs used. Because neonatology covers a short period of life, but it implies significant physiological changes. Supporting this assumption, we found a negative association between PMA and serum/saliva caffeine levels. However, the recommended caffeine dosage does not vary with maturity as in other drugs in neonatology; only some pharmaceutical guide changes the dosage schedule in the more mature infants. Even in studies with mathematical pharmacokinetic simulation models, they suggest increasing the dose of caffeine at a rate of 1 mg / kg each week of life to maintain the levels in range, despite not being carried out in routine clinical practice [18, 19]. We should also take into account the pharmacogenetics already described and the unknown information [20] about relevant polymorphisms for this drug related to clinical effects in the individualized prescription in neonates with immature metabolism of drugs, despite its renal elimination.
The option to measure non-invasively caffeine concentration by saliva sampling, will provide a free-blood and painless alternative to blood samples. Ours observed 95% limits of agreement between plasma and saliva regardless of the route of administration (-15.09; 9.20) are suitable for clinical purposes, being able to extend the indication of requesting caffeine levels. We found better correlation in O administration and at lower levels of serum/saliva caffeine concentration, as other studies [21–24].
This better correlation with oral administration can be explain by a more stable plasma levels when using this route given the excellent bioavailability of oral caffeine. But it also may be related to more mature patients receiving the oral treatment (significant differences in Fig. 3). Really, older patients eliminate caffeine faster, obtaining lower levels, and so, better correlation. It is also possible that they were more stable and probably received fewer concomitant drugs. We also have little data on pharmacological interferences in neonatology. We rule out interferences with other administered drugs using a linear regression/predictive model. In clinical practice, this better correlation reinforces the recommendation for oral caffeine use, which minimizes infectious risk and it is not related to alterations in cerebral perfusion such as rapid intravenous bolus [25].
Several studies show also a highly correlation of salivary drug concentration with serum drug concentration of caffeine in neonates (Table 2), but not in such a premature population and in the daily practice. Khanna et al. [21] reported a small study in 7 premature newborns of paired caffeine samples of plasma and saliva after oral administration. Significant correlation in caffeine concentrations determined by high pressure liquid chromatography (HPLC) were observed but only for serum levels less than 8 µg/mL which is at the bottom of recommended range therapeutic for caffeine. Moreover, samples not represented by trough samples because they were obtained four to six hours after administration.
Lee et al.[22] enrolled 59 preterm infants < 32 weeks GA in the first 7 days of life and randomized to three different IV caffeine doses (3, 15 or 30 mg/kg) and a doubled loading dose (some outside the recommended dosage). Samples were collected on days 3, 5, and 7. Saliva was collected by vacuum aspiration, a method little used in clinical practice, and caffeine were determined by HPLC. The mean ratio of saliva-to-serum concentrations was 0.924. There was a small negative bias for predictive salivary versus serum concentrations. Also, there is better correlation between serum and saliva when concentrations are lower, but the correlation remains in higher values (> 8 µg/mL) unlike Khanna et al. [21].
De Wildt et al. [23] studied the correlation between saliva and serum levels of caffeine in 140 preterm infants aged from 24 to 34 weeks GA. They compared samples (analyzed by HPLC) according to the method of the saliva collection (not stimulation, citric acid on a swab and citric acid in cheek). They suggested that saliva correlation would be better if the saliva secretion was first stimulated by citric acid, but saliva collection stimulation would be impractical in clinical practice. In addition, the volumes collected in our study (10 µL) allowed us to make several determinations of each of them, showing that it is not necessary to use salivary stimulants. In this study there was also a slightly better correlation between serum and saliva when concentrations were lower. Dobson et al. [26] reported a study of paired salivary and plasma caffeine samples in 29 preterm infants (mean GA 27.9 ± 2.1 weeks). Salivary samples were obtained using a commercially available salivary collection system when they were already stable and had reached a maturity of PMA of 33–36 weeks, when they probably no longer need analyses of caffeine level in clinical practice because of the limited clinical impact of the AOP. Their correlation coefficient between saliva and blood samples was r = 0.87. Chaabane et al.[24] published a study in 13 newborns of mean GA 32.2 ± 0.7 weeks not diagnosed of AOP (neonates with AOP were excluded). Each patient received 5, 10, 15, 20 and 25 mg/kg/day of IV caffeine from the first to the fifth day of birth. Samples were analyzed by enzyme multiplied immunoassay technique (EMIT). Caffeine concentrations showed a good coefficient of determination, r2 = 0.76. Mean difference concentrations were also better when the values were smaller (Table 2) but correlation remains in higher values. They showed that saliva caffeine monitoring by EMIT could be a valid alternative to serum in preterm infants, but not in a representative population of daily practice.
The strengths of our study are a large representative sample of a target population that benefits of this drug every day, which includes extremely preterm infants who needs IV administration and several others concomitant drug. The non-invasive collection system without salivary stimulants and the caffeine doses used according the data sheet allows the reproduction of our study to increase awareness of this widely used drug. Moreover, although the correlation is better at low doses, as in most studies, the correlation remains at higher values, which makes it useful in clinical practice before making changes to the dosage. We are also aware of its limitations. The technique used to collect samples requires a few minutes. The analysis technique is currently a research technique, however, will be possible to translate it to daily practice after its simplifying in future studies. When caffeine concentrations are needed for clinical management, normally when clinicians are concerned about subtherapeutic or toxic concentrations, they are measured in blood, but our study prove that salivary sampling may be a valid non-invasive alternative that could be used not only in these cases, but more commonly to individualize and optimize the dose of this drug until we have more data on its polymorphisms that allow us to better individualize the treatment to implement optimal precision medicine in this vulnerable population. This is especially useful when is applied to extremely low birth weight infants, in whom blood sampling must be severely restricted or when doses are raised above that established in the data sheet.