TABLE 2.

Points to consider in design, conduct, and reporting of smoking interaction studies

Topic/area	Proposed approach	Comments
Victim drugs to be examined	Drugs undergoing CYP1A1‐, CYP1A2‐, and CYP2E1‐metabolism. Also, drugs undergoing extensive glucuronidation by UGT1A6, UGT1A9, and possibly some UGT2B family members.	Note that even when the fraction metabolized of victim drugs suggests that these enzymes might be minor pathways, they can become major pathways under the condition of induction.
Study objectives	Aim for a dedicated smoking interaction study. Consider the value of modeling and simulations if applicable.	Avoid implementation of smoking interaction objectives as secondary objective in trials with other primary objectives.
Study design	Open‐label, parallel‐group design, with single‐dose administration of the victim drug.	Repeated‐dose administration may be needed for victim drugs with complex PK (in case of auto‐inhibition or auto‐induction, for example).
Study populations	Healthy adult subjects (except when not possible, for example for some oncology drugs), well‐characterized, male and female smoker and nonsmoker populations.	Proposed characterization of study populations detailed below; aim for a balanced representation of female subjects; aim for matching of subjects at least for age and sex.
Sample size	Apply a statistical (power) calculation for the estimated sample size based on the PK variability of the victim drug.	In case that a solid power calculation is not possible, enroll at least 12 subjects in each study group.
Subjects’ demographics	Document and report age, sex, body weight, BMI, and ethnicity of study subjects for each study population enrolled.	Note that sex, BMI, and ethnicity are covariates of CYP1A2 activity.
Smoker characteristics	Document and report smoking history, self‐reported current cigarette consumption per day, and the cigarette brand used. Properly define inclusion criteria for daily cigarette consumption in a manner that allows assignment of each smoker in a well‐defined category (i.e., light, moderate, and heavy smoker), when different smoker categories are planned to be enrolled. The dose of the perpetrator should aim at reaching maximum induction to allow the evaluation of the worst‐case scenario. This implies that a group of heavy smokers (smoking ≥20 cigarettes) should always be enrolled. Confirm smoker status by objective measurements (e.g., plasma cotinine concentrations).	Aim to standardize the tobacco products used by all smoking participants in the study. Note that the dose–response curve of CYP1A2 induction by smoking is steepest in the range of light smoking (i.e., significantly increases from smoking 1 to 5 cigarettes per day to smoking 5 to 10 cigarettes per day). Monitor, document, and report daily smoking requirements throughout the study.
Nonsmoker characteristics	Obtain the self‐reported nonsmoker history. Inquire subjects for sources of second‐hand smoke exposure (e.g., smokers living in the household, etc.). Define minimum time‐period of nonsmoker status before study start. Confirm nonsmoker status by objective measurement (e.g., by quantification of plasma cotinine concentrations).	Currently proposed cutoff to distinguish between smokers and nonsmokers is proposed with 3 ng/ml (Benowitz et al., 2009). 43 Take care of spatial separation of smokers from nonsmokers at all study days.
Identity of perpetrator tobacco products	Define, document, and report the perpetrator tobacco products that are to be used throughout the study, and aim for standardization across subjects to the extent possible.	It is acknowledged that it might be a challenge to convince study participants to change their cigarette brands for the study period, but one should aim for this. It might have ethical implications when subjects are asked to switch from light cigarettes to brands containing more nicotine. This needs to be considered.
Information on smoking requirements or restrictions during study conduct	Provide adequate and specific information on smoking restrictions or requirements during study conduct.	Close monitoring of adherence with the protocol‐specified restrictions need to be conducted and documented.
Concomitant medications	Apply strict requirements regarding prohibited concomitant medications, including OTC products, herbals, vitamins, and other supplements. Pay particular attention on the prohibition of CYP1A2‐ or multiple CYP‐enzyme inhibitors and inducers.	There are only few CYP1A2 inducers known among marketed medicinal products (e.g., omeprazole). Oral contraceptives, fluvoxamine, ciprofloxacin, enoxacin, sertraline, methoxsalen, mexiletine, and vemurafenib are known CYP1A2 inhibitors of varying potencies. A complete list of marketed CYP1A2 inhibitors should be reviewed and addressed in the exclusion criteria of the study protocol.
Dietary and other lifestyle restrictions	Cruciferous vegetables (e.g., broccoli and cauliflower), caffeine and charcoal grilled meat/foods have been shown to induce CYP1A2 activity, whereas apiaceous vegetables (e.g., carrots and celery) and grapefruit juice are reported to inhibit CYP1A2 activity. Foods and beverages containing these aliments need to be prohibited. Alcohol consumption should also be prohibited a certain time period before and throughout the study. Unaccustomed strenuous physical exercise has been shown to induce CYP1A2 activity and thus, should also be prohibited throughout the study.
CYP1A2 activity assessment/phenotyping	For the interpretation of the PK outcomes of CYP1A2 victim drugs, a CYP1A2 activity assessment by a validated phenotyping method in smoker and nonsmoker populations is recommended.	Caffeine is the most often used CYP1A2 phenotyping probe because of the predominant role of CYP1A2 in its overall metabolism, and the well‐established safety and tolerability. The assessment of the oral caffeine clearance after administration of a single 200 mg caffeine tablet is considered the current gold standard.
Pharmacogenetics of study populations	Consider to genotype study participants for polymorphisms known to alter activity and/or inducibility of CYP1A2.	As this is a still evolving area, apply current knowledge based on literature search. Altered inducibility of CYP1A2 may be also due to genetic variations in the regulation of the CYP1A2 gene (e.g., AhR or by nongenetic factors modulating the intracellular inducer concentrations (e.g., cellular efflux by P‐gp etc.).

Abbreviations: AhR, aryl hydrocarbon receptor; BMI, body mass index; OTC, over‐the‐counter; PK, pharmacokinetic.