Infrared pupilometry as a biomarker of drug effects

Summary

Introduction:
Measurement of the size of the pupil is used as a biomarker of drug efficacy, mainly their affecting central nervous system. The aim of our study was to evaluate sensitivity of various pupilometric parameters as biomarkers of widely used opioid analgesic drug tramadol.

Methods and results :
Pharmacodynamic action of tramadol drops given orally in standardized dose of 0.7mg/kg was studied in 60 healthy volunteers. Commercially available infrared pupilometer “Pupilscan II (™)” was used for the measurements of static and dynamic pupilometric parameters prior the dosing and 2.5 hours afterwards. Drug-induced decreases of the initial diameter (0, 49 mm) a final diameter (0, 38 mm) were significant in the right eye, as well as in left eye. Minimal parameters ( 0,35 mm) and time to minimum (0,03 ms) were significantly lower after tramadol administration in the left and right eye only.

Summary:
Our results confirm the use of pupilometric as an objective, non-invasive tool for evaluation of pharmacodynamic activity of drugs.

Key words:
pupilometry – tramadol – biomarkers – pupilary response

INTRODUCTION

Biomarkers of drug efficacy are of importance in clinical drug development of various types of agents including drugs affecting central nervous system with known prolonged onset of therapeutic effect (1). Appropriately chosen biomarker should be related to drug mechanism of action and the relationship to clinical endpoints should be clearly defined. PK/PD relationship represents an important part of such a research. A well defined biomarker can be helpful for decision-making process in early phases of clinical development. Some biomarkers may be also used for identification and analysis of individual responsiveness or resistance to therapy. Pharmacogenetic biomarkers are already used to predict therapeutic response in some drugs and to select optimal dosing, although the examples are seldom in clinical settings.

Pupilary diameter measurement is a traditional method used in clinical drug development, especially as an indicator of drug action on central nervous system. Adrenergic stimulation causes mydriasis by contraction of m. dilatator pupilae. Cholinergic stimulation, on the contrary, leads to contraction of m. sphincter pupilae resulting in subsequent miosis. Pharmacodynamic effects of opioid compounds are mediated not only via opioid receptors, but may also interact with monoaminergic system.

The aim of our study was to evaluate sensitivity of various pupilometric parameters as biomarkers of widely used opioid analgesic drug tramadol, which acts through both opioid and non-opioid mechanisms.

METHODS

Pharmacodynamic action of tramadol ( Tramal por. gtt. sol., Gruenenthal GmbH, Aachen, SRN) drops given orally in standardized dose of 0.7 mg/kg b.w. was studied in 60 healthy volunteers (26 males and 34 females). The drug was administered in the morning with 150 ml of water. Mean age ± SD of the volunteers was 23.2 ± 4.9 years. Pupilometric examination of both eyes was conducted in darkened, quiet, well heated room after 5 minutes of adaptation to the dark environment. Commercially available infrared pupilometer “Pupilscan II ^(™)” was used for the measurements prior the dosing and 2.5 hours afterwards.

The study was approved by the Ethics Committee of the General Teaching Hospital in Prague and by the State Institute for Drug Control. All volunteers signed their informed consent prior to the participation in the study. Figure 1 shows pupilometric parameters describing initial diameter of the pupil (INIT), minimal diameter (MIN), last observed pupilary diameter (FINAL) and time to minimum diameter (TTM) that were evaluated in our study. Reflex amplitude (RA) was computed as a difference between INIT and MIN values.

Statistical significance was considered at 5% level and the difference between values before and after dosing was compared using Wilcoxon´s test for dependent samples.

RESULTS

Table 1 depicts pupilometric parameters before and after tramadol administration. INIT and FINAL values decreased significantly in both eyes, while MIN and TTM were significantly lower only in left and right eye, respectively. Changes of all other dynamic parameters did not reach level of statistical significance. We have observed considerably large inter-individual variability in pupilary reactions. For example, large inter-individual variability is illustrated by a box plot in the Figure 2 depicting difference of INIT before and after treatment administration. No miotic reaction was observed in almost one third of the volunteers, while miotic reaction above upper quartile was seen in one fourth of the subjects. Observed values deviate from normal distribution with skewness of 5.2 and kurtosis of 9.1. Diameter differences were not statistically dissimilar when compared left and right eyes.

DISCUSSION

Infrared pupilometry is currently a preferred method for evaluation of pupilary diameter in clinical research ( 2, 3 ). CCD cameras monitor pupil of the eye, which is illuminated by infrared light emitting diodes from a constant distance.

Standard pupilometers have also visible light emitting diodes for a stimulus leading to pupil constriction and enabling dynamic measurement of pupilary reflex.

We have previously evaluated possibility of infrared pupilometry by standard digital photography, which provides opportunity for static pupilometric measurement without having special pupilometer (4). In this article, we report methodological observations after administration of tramadol using pupilometer Pupilscan II. Miotic effect of tramadol was confirmed in static and dynamic pupilometric parameters. Mean miotic effect of tramadol in our study is in the range that was reported previously (5, 6, 7). The most pronounced tramadol-induced miosis was seen in the INIT, MIN and FINAL parameters. However, other dynamic parameters displayed high inter-individual variability.

Opioid and non-opioid effects of tramadol and its O-demethylated metabolite interact with physiological mechanisms regulating pupil size in man. Number of observations of tramadol-induced miosis has been reported previously (8, 9, 10). These observations suggest that pharmacodynamic effects of tramadol are mediated by synergistic effect of tramadol and O-desmethyltramadol that is formed in liver via CYP2D6. Opioid action is mainly attributed to the O-demethylated metabolite, since it possesses approximately 200fold higher affinity to μ-opioid receptors. In comparison with full  μ-agonist opioids, the miosis induced by tramadol was of a smaller extent. The mean maximum decrease of the pupil size after 10 mg of morphine was approximately 1.5 mm compared with baseline (11).

Infrared pupilometry was also used to study effects of other opioid action of other analgesics and it is currently considered to be an objective measurement of efficacy mediated through μ-opioid receptors (12). For example, buprenorphine, partial agonist of opioid receptors, induced miosis in human volunteers (13). Peackock et al. documented correlation between plasma codeine levels and pupil size (14). Further, heroin induced euphoria was related to heroin induced mioisis (15). Doses and plasma levels of drugs inducing miosis were considerably higher in patient with dependence as compared with people without heroin dependence. The ratio of heroin plasma levels to miosis may be therefore one of possible biomarkers of dependence. Pupilometric evaluation of opioid action of drugs might be used as surrogate endpoint in pain treatment.

Pupilometry has also been used as biomarker for serotonin agonists’ effects (16) as well as antihistaminics and diazepam (17) .

Our results confirm pupilometry as an objective, non-invasive tool for evaluation of pharmacodynamic activity of drugs.

ABBREVIATIONS

• INIT – initial pupilometric parameters
• MIN – minimal pupilometric parameters
• FINAL – final pupilometric parameters
• RA – reflex of amplitude
• TTM – time to minimum
• CYP 2D6 – enzyme 2D6

ACKNOWLEDGEMENTS
The authors would like to thank Michael Friedrich Bőttcher (Bayer AG Wuppertal, Germany) for Pupilscan II. This study was supported by grants No. VZ MSM 0021620849 and VZ MSM 0021620820.

CORRESPONDING AUTHOR
MUDr. Olga Matouskova
Institute of Pharmacology, Charles University, Prague, 1st Faculty of Medicine
Albertov 4
Prague 2
128 00
e – mail : olga.matouskova@cuni.cz
phone number : 00420224968140