Pregabalin was most commonly prescribed for the treatment of pain in PHN (30.5%) and fibromyalgia (18.4%) in Taiwan, and this was also reflected by a high proportion of concomitant prescription of other analgesics (table 1). Most of the prescriptions were limited to short-term use, as evidenced by a mean duration of prescription lasting 149.5±299.8 days (median 28, interquartile range [IQR] 14-118), especially in PHN (89.4±224.1 days) and fibromyalgia (150.8±302.8 days). Besides, the mean dose prescribed was 140.7±92.8mg, which was lower than the doses used in clinical trials for NeP (3) and fibromyalgia (8), although the mean dose of prescriptions for epilepsy could be within the lower range of recommended doses (10).
The finding that pregabalin prescriptions were typically of short duration is in keeping with prior reports. According to a retrospective administrative claims data analysis from the United States, only 56.9% of patients initiating pregabalin for PHN obtained a refill during the one-year follow-up period (16). Similarly, 34% had just one single dispensed prescription for various indications, including epilepsy, generalized anxiety disorder, and neuropathic pain, indicating only 64% had further refills after the first prescription in Sweden (17). Besides, according to an Israeli study in fibromyalgia, the median time to discontinuation for antiepileptics, namely pregabalin and gabapentin, was 30 days (IQR 30-106 days) (18), and the mean prescription duration of pregabalin for pain was 52.2 days in a Japanese retrospective real-world study (19). In addition, similar findings were reported in prospective real-world studies involving patients with NeP. The median durations of pregabalin use were 84.0 days in a Latin American multicenter, open-label study (20), and 62 days in a Greek non-interventional, multicenter study (21). In the present study, the persistence rates were 29.3% and 12.1% at 90 days and one year, respectively, indicating only a minority of patients continued to use pregabalin. All the above findings indicate that pregabalin prescriptions are usually discontinued early, which is a phenomenon commonly seen in different parts of the world.
A number of factors could be responsible for the short duration of pregabalin treatment for pain. As the NeP in PHN could resolve in months or years (22, 23), treatment would be no longer necessary after resolution of symptoms. In fact, the prevalence of PHN was 11.6%, 8.5%, 7.4%, and 6.0% at months 3, 6, 9, and 12 following herpes zoster (22), and the natural course of PHN could be one of the major causes for the short-term use of pregabalin in that patient population. More importantly, pregabalin could be prescribed off-label for the acute pain in herpes zoster rather than for PHN in some patients in the present cohort. In fact, 5.8% of pregabalin users were prescribed with anti-viral agents at the same time (Table 1), and concomitant use of anti-viral agents was associated with decreased likelihood of pregabalin use at one year (OR 0.41, 95% CI 0.35-0.47, p<0.001) (Table 4). Although pregabalin is only indicated for PHN, there is some evidence supporting its efficacy for acute pain of herpes zoster (24, 25). On the other hand, pregabalin could also be discontinued due to a lack of desired efficacy or the development of intolerable side effects. In fact, adverse effects of pregabalin treatment usually appear early in the treatment course, and usually resolve after prolonged use (11). Premature discontinuation preclude further escalation of the dosage, and thus could also account for the finding that most prescriptions were under-dosed, especially in pain disorders. Both could lead to poor treatment outcomes and hence compromised quality of life. An in-depth understanding of the dynamics of development of adverse events, would be helpful to optimize the prescription of pregabalin.
Among the different indications included, epilepsy patients had the greatest likelihood of long-term use, and PHN the least. Similar findings were reported in the Swedish study mentioned above (17), in which epilepsy and NeP had the best and worst persistence rates during follow-up. Generally speaking, studies evaluating drug treatment in epilepsy usually report high persistence or adherence rates (26-28). This could probably be attributed to the unremitting nature of most seizure disorders, as well as the devastating and potentially life-threating consequences of not receiving treatment. In contrast, a substantial proportion of PHN resolves within months (22, 23), and consequently, treatment for pain would be no longer necessary at that time. Taken together, the durations of the underlying diseases could have an important impact on the durations of pregabalin prescriptions. Besides, as in the American study mentioned above, the low persistence rate of pregabalin use in the treatment of NeP could also be attributed to suboptimal dosing, with 87 % reaching ≥ 150 mg/day and 27 % reaching ≥ 300 mg/day (16), which were higher than the corresponding figures of 62.5% and 6.4%, respectively, in the present study (Table 3). In fact, most of the prescriptions conformed well to the treatment recommendations. However, more than a third of the prescriptions for pain control were <150 mg/day, which was below the lowest recommended dose, particularly in prescriptions for DPNP (44.1%) and fibromyalgia (39.5%) (table 3). Besides, the average doses prescribed for NeP (147.2±100.3 mg for PHN, 131.2±93 mg for DPNP) (Table 3) were lower than those in the American (187 mg for PHN) (16) and Swedish studies (275 mg for NeP) (17). In comparison, the findings were variable in prospective open-label studies. Although the doses prescribed fell in the lower range of the recommended doses in some studies, the doses were still higher than those in the current study, possibly with the exception of the Greek study, in which the most commonly prescribed dose was 150 mg/day (21). On the other hand, the mean daily dose was 301 mg in a German open-label multicenter study for patients with DPNP and PHN (29). Besides, the average doses in the first, second, and third month and after three months of treatment for NeP were 148.4, 145.3, 169.3, and 240.7 mg/day, respective, in a Danish study of peripheral NeP (30), and 42.6% and 38.9% of patients achieved a daily dose of 300 mg and 600 mg, respectively, in the Latin American NeP study (20). The exact cause why the average dose used in the current study was lower than in some earlier reports was unknown. It could be possible that some patients could benefit from such low doses, which made further titration unnecessary. This might be the case for DPNP, as pregabalin prescriptions in such patients were characterized by a long duration (221.9±330.1 days) (table 2) despite a relatively lower dose (131.2±93 mg) (table 3). However, it could also be possible that higher doses of pregabalin were not that well tolerated in some patients. In such cases, there could be inadequate therapeutic effects at lower doses, although dose escalation might be precluded by side effects. In addition, ethnic factors could also play a role. The tolerability of pregabalin was reported to be comparable to, or slightly worse than, that of placebo in clinical trials (10, 13-15). which were mostly carried out in North America and/or Europe. In fact, side effects, such as dizziness, somnolence, peripheral edema, and weight gain, were more common among Asians as compared to Caucasians (12). This could result from increased exposure to pregabalin due to the relatively lower average body weights among Asian patients, which could result in decreased tolerability, and possibly lower average doses prescribed. On the other hand, although ethnicity did not seem to have a major impact on the pharmacokinetics of pregabalin (31), it remains possible there could be ethnic differences in the pharmacodynamics.
The most important strength of the present study is the large-scale sample recruited, and the almost complete coverage of our NHI leads to the relative freedom from selection bias of the dataset. Besides, the data from NHIRD have been validated by a number of studies, and have yielded important findings that are widely accepted (32-34). On the other hand, the main weaknesses include the validity of the diagnosis coding and the lack of relevant clinical information regarding the initiation and discontinuation of the prescription. PHN, DPNP, and fibromyalgia could not be accurately coded in the ICD-9-CM coding system, and this remains an issue in the ICD-10 coding system, except for fibromyalgia. Besides, it could be possible that fibromyalgia could be miscoded as other musculoskeletal diseases. In addition, as discussed, some of the patients categorized as PHN could actually be treated for acute pain in herpes zoster. However, when these diagnosis codes were associated with pregabalin prescriptions concomitantly, the probability that these codes correspond to the designated diagnoses should be high. In addition, the exact cause for treatment discontinuation could not be ascertained as such information is not available in the NHIRD. In fact, this is an intrinsic weakness for studies based on administrative claims databases of health insurance or drug prescription register (16-18), and it is unknown whether the discontinuation of prescriptions of pregabalin was attributed to either efficacy or tolerability issues. Moreover, based on the methodology of the current study, medications prescribed on the same day would be categorized as concomitant, even though the prescriptions might not be necessarily given at the same consultation or even from the same physician. Although the medications could be adjusted or changed within the same day, the underlying diagnoses would remain the same. Therefore, such a methodological issue would probably have little impact on the analysis. Despite the above weaknesses, the present study could still provide some useful information regarding the pattern of pregabalin prescriptions in our country.