Nicotine & Tobacco Research Volume 6, Number 5 (October 2004) 865–867
Letter
Investigating subgroups in smoking cessation treatment response: Response to Perkins Marcus R. Munafo`, Mike Bradburn, Lucy Bowes, Sean David
To the Editor: The interest generated in our original article (Munafo`, Bradburn, Bowes, & David, 2004) reflects the growing interest in individual differences in smoking behavior and response to treatment. This is an encouraging trend, as it is becoming clear that there are likely to be gains made by tailoring existing treatment regimens to individuals on the basis of factors such as sex, as well as genotype (e.g. Walton, Johnstone, Munafo`, Griffiths, & Neville, 2001), readiness to quit (e.g. Dijkstra, De Vries & Roijackers, 1999), and so on. Although the efficacy of both nicotine replacement therapy (NRT) and bupropion for smoking cessation is now well established, there is substantial variability in treatment response (Swan, Javitz, Jack, Curry, & McAfee, 2004), and the prediction of this variability is of both clinical and theoretical importance. Our recent meta-analysis attempted to address this issue with respect to the possibility of sex differences in response to NRT patch treatment. Many of the points highlighted in the commentary on our original article by Perkins (2004) reflect the relative paucity of reported outcomes by sex (and other variables that could be used to identify individual differences in response) in the extant literature. As Perkins points out, only one of the 33 studies included in the meta-analysis reported outcome data separately for men and women. There are likely to be many factors contributing to individual variation in Marcus R. Munafo`, Ph.D., Lucy Bowes and Sean David, M.D., Cancer Research UK General Practice Research Group; Mike Bradburn, M.Sc., Cancer Research UK Centre Medical Statistics Group, University of Oxford, UK. Correspondence: Marcus Munafo`, Ph.D., CR-UK GPRG, Department of Clinical Pharmacology, University of Oxford, Oxford OX2 6HE, UK. Tel.: z44 (0)1865 224807; Fax: z44 (0)1865 224989; E-mail:
[email protected]
response to NRT (and other pharmacotherapies), and meta-analytic techniques only allow the investigation of factors as they are reported in the literature. In a separate project, the authors of the original article attempted to address the question of differences between ethnic groups in NRT response but failed because of a lack of available data. Moreover, as Perkins also notes, variability in procedures across trials can contribute to between-study variance, which may limit the extent to which data from such trials can be combined. We explicitly restricted our analysis to studies of NRT patch efficacy for this reason, thereby removing the potential confound of behavioral differences in administration, with 6-month or greater follow-up. Perkins (2004) details two studies that may have contributed to our analysis and that appear to support the existence of sex differences in response to NRT patch. Unfortunately, although our search strategy identified the trial by Hurt and colleagues (1994), it did not capture the related report detailing the development of the Self-Administered Nicotine Dependence Scale (SANDS) by Davis et al. (1994), which did report cessation data separately for men and women. The corresponding author on the original clinical trial report (Hurt et al., 1994) replied to our request for the release of data grouped by sex, indicating his regret that he lacked the resources to extract that information from the archived data (R. D. Hurt, personal communication, July 31, 2002). It is unfortunate that our search strategy did not capture the Davis et al. (1994) report, but this paper reports the psychometric properties of the SANDS and is not included in the Cochrane Review (Silagy, Lancaster, Stead, Mant & Fowler, 2002) that we used to identify studies for inclusion.
ISSN 1462-2203 print/ISSN 1469-994X online # 2004 Society for Research on Nicotine and Tobacco DOI: 10.1080/14622200412331295942
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RESPONSE TO PERKINS, INVESTIGATING SUBGROUPS
The paper by Wetter and colleagues (1999) reports data combined across three trials, and it was not clear whether these trials were included elsewhere in our meta-analysis. Attempts to contact the corresponding author of this paper were unsuccessful. One of the trials included in Wetter et al. (1999) was certainly captured by our search strategy (Fiore et al., 1994), but the corresponding author on this report replied to our request for data release with an explanation that the original data are no longer available (M. C. Fiore, personal communication, August 7, 2002). A real danger in meta-analytic studies is the inclusion of the same dataset more than once (Munafo` et al., 2003), and we were therefore obliged not to include the Wetter et al. (1999) study in the absence of confirmation that the data reported therein were not included elsewhere in our analysis. If we were to include these data in our analysis, the odds ratio (OR) of quitting at 6 months for men on NRT patch vs. placebo would be 2.09 (95% confidence interval [CI]~1.65–2.66, pv.001) and in women would be 1.56 (95% CI~1.24–1.98, pv.001). Critically, the ratio OR comparing the effect at 6 months in men vs. women would be 1.35 (95% CI~.97–1.90, p~.08) and therefore would remain nonsignificant, although the combined effect would constitute a trend. However, we are unclear as to whether these data comprise all of the additional information available to Perkins (2004), or were selected specifically to illustrate the possibility of a sex difference in NRT patch efficacy. It is by no means clear that if other data that could not be included in our meta-analysis were to be included that this trend would continue to strengthen. Such problems are typical of meta-analytic studies, and, as Perkins (2004) notes, to be able to address the question comprehensively requires the reporting (or availability) of data that are similarly comprehensive. It is not a trivial matter to retrospectively identify archived data from a study that may have been conducted several years previously. This difficulty is particularly acute in the case of industry-sponsored trials in which the custodian of the original data often is unclear, a point we highlight in our original paper. The lack of reporting of NRT efficacy separately for males and females presumably occurs for three reasons. First, it is possible to imagine many factors that may potentially interact with treatment, such as patient demographics (e.g., ethnicity), physical or mental status (e.g., affective disorder), genotype (cf. Yudkin et al., 2004), and level of dependence (e.g., cigarettes smoked per day). Second, the individual studies often were quite small, with most reporting fewer than 50 successful quit attempts in the entire study. Third, any associations found may well be dismissed by either referees or readers as spurious findings resulting from data dredging. Trials therefore often have insufficient statistical power and theoretical
justification to explore the large number of potential subgroups, a point highlighted by Perkins (2004), and in practice have tended to present only an overall effect, often rightly so. At the time when the trials included in our meta-analysis were designed, the theoretical justification for studying sex differences in treatment response may not have been sufficiently strong to justify the appropriate subgroup analyses. The question of sex differences in treatment response is one that now has theoretical grounds, however, and is supported by a certain amount of preclinical data (e.g., Perkins, Jacobs, Sanders, & Caggiula, 2002). Nevertheless, it is worthwhile to take from this study the message that sex alone does not appear to have a dramatic effect on NRT patch efficacy, that women do derive substantial benefit from NRT patch treatment for smoking cessation, and that elucidating the mechanisms underlying perhaps modest differences in efficacy should involve using more complex data sets and, in particular, studies designed explicitly to explore such individual differences in treatment response (with a concomitant improvement in the reporting of outcomes by sex and other variables). As Perkins (2004) comments, addressing individual differences requires sufficient statistical power to detect group6treatment interactions only evident in very large studies. The power analysis that he presents highlights this point extremely well, and it is possible that differences in treatment response may in fact be smaller than the doubling of efficacy assumed in the power calculation reported. The requirement for sufficient power to test interactions may necessitate large-scale, multicenter studies explicitly designed to recruit large samples for this purpose. This also reinforces the suggestion we make that data archiving would enable a large corpus of data to be built up for the purposes of such analyses. This also would require the standardization of procedures across trials, which is already becoming increasingly common (e.g., the use of long-term, biochemically validated continuous abstinence as the criterion for quitting). Perkins (2004) also raises the interesting point that a more clinically relevant comparison may be between different treatments, so that interactions between group (e.g., sex) and treatment type (e.g., NRT patch vs. inhaler) are explicitly examined, possibly without the need for a placebo arm. This highlights the fact that what is required is not a traditional clinical trial of treatment efficacy but an explicitly designed study of group6 treatment interactions, adequately powered to test these interactions. In summary, we welcome debate that brings attention to the relative paucity of research that addresses the general question of individual differences in response to smoking cessation treatment, in particular if this leads to improved reporting of subgroup effects and group6treatment interactions, and the design of future
NICOTINE & TOBACCO RESEARCH
studies to address these questions directly. On the basis of existing knowledge, we regard it premature to suggest that large sex differences exist in response to the NRT patch. However, sex differences regarding other NRT delivery devices for which behavioral differences in administration may be more likely to influence treatment response, and other pharmacotherapies, remains unresolved.
References Davis, L. J., Hurt, R. D., Offord, K. P., Lauger, G. G., Morse, R. M., & Bruce, B. K. (1994). Self-administered nicotine-dependence scale (SANDS): Item selection, reliability estimation, and initial validation. Journal of Clinical Psychology, 50, 918–930. Dijsktra, A., De Vries, H., & Roijackers, J. (1999). Targeting smokers with low readiness to change with tailored and nontailored self-help materials. Preventive Medicine, 28, 203–211. Fiore, M. C., Kenford, S. L., Jorenby, D. E., Wetter, D. W., Smith, S. S., & Baker, T. B. (1994). Two studies of the clinical effectiveness of the nicotine patch with different counseling treatments. Chest, 105, 524–533. Hurt, R. D., Dale, L. C., Fredrickson, P. A., Caldwell, C. C., Lee, G. A., Offord, K. P., Lauger, G. G., Marusic, Z., Neese, L. W., & Lundberg, T. G. (1994). Nicotine patch therapy for smoking cessation combined with physician advice and nurse follow-up: One-year outcome and percentage of nicotine replacement. Journal of the American Medical Association, 271, 595–600.
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