Figure 1 (a) shows a 3d representation of a hollow microneedle array. The component needles are 1 mm in length and are arranged in a 3 x 3 array; the needle pitch, the distance between the centers of the needles, is 3.5 mm. Figure 1 (b) shows a side view optical micrograph of a single microneedle within the hollow microneedle array, and Fig. 1 (c) shows a top view optical micrograph of a single microneedle within the hollow microneedle array. The needle outer diameter is 250 µm, and the needle inner diameter is 150 µm. Figure 2 (a) shows a scanning electron micrograph of single microneedle within the hollow microneedle array, Fig. 2 (b) shows a scanning electron micrograph of the tip of a single microneedle within the hollow microneedle array, and Fig. 2 (c) shows a top view scanning electron micrograph of a single microneedle within the hollow microneedle array. The needle exhibits a circular bore and a sharp tip.
The plasma drug concentrations for naloxone and nalmefene from each route of administration to dogs are shown in Fig. 3 (top and bottom panel). As observed in the figure, the microneedle injection produced high plasma naloxone concentrations rapidly that were higher than the IM injection of the same dose, but the absorption was more delayed and lower from the nalmefene microneedle injection. The duration of detectable concentrations from the nalmefene injection was much shorter, compared to the naloxone injection because the assay limit was reached quickly at these low nalmefene concentrations. As seen by the error bars on the figures, there was considerable variability in the concentrations for both drugs.
The pharmacokinetic results are shown in Tables 1 and 2. For the naloxone injection, the peak was higher from the microneedle injection, as seen by the CMAX values, with the microneedle injection producing a peak that was a mean of 2.15 x higher than the IM injection. The extent of absorption, shown by the area under the curve (AUC) values was not as high from the microneedle injection, which was likely due to the shorter half-life (mean of 0.46 hr and 1.2 hr from the microneedle and IM injection, respectively).
Table 2
Pharmacokinetic parameters after injection of nalmefene to dogs at a dose of 0.014 mg, either intramuscularly or by hollow microneedle array.
|
Intramuscular
|
Microneedle
|
Parameter
|
Units
|
Mean
|
Std. Dev.
|
Mean
|
Std. Dev.
|
AUC (0 to Cn)
|
hr*ng/mL
|
6.72
|
2.73
|
4.72
|
2.64
|
AUC (0 to infinity)
|
hr*ng/mL
|
8.02
|
3.61
|
5.08
|
2.71
|
CMAX
|
ng/mL
|
4.71
|
1.18
|
3.80
|
2.64
|
Half-life (t½)
|
hr
|
1.39
|
1.18
|
0.85
|
0.59
|
Terminal rate
|
1/hr
|
1.01
|
0.80
|
1.09
|
0.56
|
MRT
|
hr
|
2.00
|
1.51
|
1.59
|
0.97
|
TMAX
|
hr
|
0.33
|
0.17
|
0.45
|
0.43
|
Fraction CMAX
|
|
|
|
0.94
|
0.75
|
Fraction AUC
|
|
|
|
0.58
|
0.12
|
Legend: AUC (0 to Cn), area under the curve from time 0 to the last measured time point (Cn); AUC (0 to infinity), the area under the curve from time 0 to infinity; CMAX, maximum (peak) plasma concentration; half-life (t½) terminal half-life; MRT, mean residence time; TMAX, time to peak (CMAX) concentration; Fraction CMAX, the ratio of microneedle CMAX concentration to intramuscular CMAX concentration; Fraction AUC, the the ratio of microneedle AUC to intramuscular AUC concentration.
For the nalmefene injection the peak (CMAX) was slightly lower from the microneedle injection with a mean difference of 0.94x lower, compared to the IM injection. The extent of absorption was considerably lower from the microneedle injection, with a mean relative fraction absorbed of 0.58. Compared to naloxone, the nalmefene concentrations were proportionally much lower for nalmefene. Although the dose of naloxone was approximately 3x higher than the nalmefene dose, the difference in CMAX was over 12x higher for naloxone after the microneedle injection and approximately 5x higher after the IM injection. The reasons for these differences are undetermined. According to the NIH U.S. National Library of Medicine (https://chem.nlm.nih.gov/chemidplus/) naloxone and nalmefene have similar lipid solubility values (LogP) but naloxone is much more soluble than nalmefene by a factor of approximately 10x. For a drug to be readily absorbed into plasma after an injection, solubility is an important factor and it is possible that this accounts for the observed differences in relative plasma concentrations from the nalmefene injections compared to naloxone injections. In a study in people [33] after a nalmefene hydrochloride injection of 0.019 mg/kg (mean) IM, the CMAX was 1.53 ng/mL at a TMAX of 0.33 hours. Thus, the IM absorption of nalmefene we observed in dogs was actually higher than reported for people.
This is the first study to examine the disposition of nalmefene and naloxone in dogs delivered with a hollow microneedle array. The objective of this study was to characterize absorption, using the dog as our model, for future development of a hollow microneedle array that will deliver naloxone or nalmefene to humans for emergency treatment of opioid-induced respiratory depression (OIRD).
We used the dog as our model because they will be the basis of our future assessment. The dog is an ideal model animal in these studies because (a) they can be easily instrumented and sampled for pharmacokinetic studies, (b) their heart rate, respiratory rate, blood oxygenation, and other parameters are easily monitored during assessment of the hollow microneedle arrays, (c) they have a predictable response to an overdose of fentanyl [11], and (d) their skin stratum corneum thickness is similar to the forearm of humans, thus providing an accurate assessment of intradermal delivery with these microneedles that are 1 mm in length. Our laboratory at the College of Veterinary Medicine has extensive experience handling and instrumenting dogs for pharmacology studies.
We selected naloxone for our studies because it is the standard method of opioid overdose emergency treatment [2]. We showed that the microneedle injections of naloxone produce rapid plasma drug concentrations in dogs compared to a traditional IM injection at the same dose with comparable rate of absorption (Fig. 3, Table 1). The dose we used (0.04 mg/kg) is the standard dose in dogs for opioid reversal [12]. We selected nalmefene because it had potential for future development owing to its 4-5x greater potency and 2x longer duration than naloxone [11–13]. The dose used in our study (0.014 mg/kg) was lower than the naloxone dose to account for its higher potency. However, we did not anticipate the lower relative absorption from the nalmefene injection, which may limit the potential for this application in the future.
The mean peak concentrations (CMAX) were 2.15x higher from the microneedle injection of naloxone compared to the IM injection, but the mean relative bioavailability (measured by AUC ratios) was less (by 0.85x) for the microneedle injection (Table 1). This was likely due to a faster half-life associated with the microneedle injection. For rapid treatment of opioid overdose, a rapid high peak concentration is more critical to a successful outcome than the AUC.
We observed substantial variation in on our data as seen by the error bars in Fig. 3 as well as the variation in Tables 1 and 2. This result may be associated with our observation that some of the dogs pulled away from the microneedle injection because of a slight “sting” at the time of injection; this reaction may have produced an incomplete or variable injection volume. We attribute this response to the formulation of each drug. Both drugs are formulated as hydrochloride salts (HCl). It has been our observation from clinical use of medications in dogs that drugs formulated in hydrochloride can cause an initial sting or discomfort upon injection because of the low pH. We do not anticipate such a problem when used to treat an opioid overdose because the individual will be under the strong influence of a potent opioid at the time of treatment.
Pharmacokinetic data for naloxone or nalmefene in dogs is somewhat limited. Naloxone is characterized by a short half-life and a high hepatic clearance that exceeds hepatic blood flow [28]. Pace et al. studied naloxone at a high dose of 5 mg/kg IV in dogs; however, they used a radioimmunoassay, which has considerable cross-reactivity with other opioids and metabolites. In that study, they described a terminal half-life of approximately 71 minutes [31].
Other studies used a more specific HPLC assay with electrochemical detection or HPLC coupled with mass spectrometry (LCMS), as performed in this study. Garrett et al. used an electrochemical detection approach and showed that there was no pharmacokinetic dose-dependency when they compared 10-fold differences in doses in dogs of 0.47 and 4.7 mg/kg IV [32]. The terminal half-life (mean) was 55.6 minutes with a high clearance. Wahler et al. showed a terminal half-life (mean) of 37 minutes from an IV injection of 0.04 mg/kg, the same dose used in our study [28]. When they gave the same dose intranasal, the bioavailability was only 32%. Our study also showed that the disposition of these drugs in dogs was characterized by a rapid terminal half-life in most dogs (Fig. 1, Tables 2 and 3) with a mean of 1.2 hr for naloxone given IM and 0.46 hr after the microneedle injection. Therefore, these values are in a similar range as other studies. Nalmefene also had a rapid half-life of approximately 1 hr for both routes of administration. The measureable concentrations were also short-lived because the values quickly fell below our assay limits.
Other than the slight discomfort from the microneedle injection, as noted above, there were no adverse effects observed in the dogs after these injections. We observed that the microneedle array produced a slight imprint on the clipped skin of these dogs; this feature was only temporary and resolved a few hours after injection.