New questions regarding bioequivalence of levothyroxine preparations: a clinician's response

Levothyroxine sodium pentahydrate tablets - formulation considerations

Even though levothyroxine sodium pentahydrate tablets have been in the market since 1955, there continue to be recalls due to sub potency. We have comprehensively reviewed the factors affecting its stability in solid oral dosage forms. A compilation of marketed formulation compositions enabled the identification of the potential 'problem excipients'. Two excipient properties, hygroscopicity and microenvironmental acidity, appeared to be responsible for inducing drug instability. In drug products, depending on the formulation composition and storage conditions, the pentahydrate can dehydrate to highly reactive levothyroxine sodium monohydrate, or undergo salt disproportionation to the free acid form of the drug. The USP assay method (HPLC based) is insensitive to these different physical forms of the drug. The influence of physical form of levothyroxine on its chemical stability is incompletely understood. The USP has five product-specific dissolution tests reflecting the complexity in its evaluation.

Comparison of Safety Profiles of the New and Old Formulations of Levothyroxine in a First Global Introduction in France

BACKGROUND

Levothyroxine sodium marketed in France was reformulated following a French National Agency for Medicines and Health Products Safety request for a more stringent potency specification. Despite previously established purity and bioequivalence of the new and old formulations, reports of adverse events substantially increased following reformulation. This analysis evaluated the nature and relevance of the medically confirmed safety reports.

METHODS

Spontaneous and solicited individual case safety reports in France were retrieved from 26 March 2015 to 30 June 2016 (old formulation) and 26 March 2017 to 30 June 2018 (new formulation). Rates of reports and adverse events were calculated for the overall patient population and for at-risk subgroups. Adverse events delineated by thyroid-stimulating hormone levels were evaluated.

RESULTS

A total of 295 and 42 775 reports for the old formulation and new formulation, respectively, were retrieved, with 149 and 5503 medically confirmed. The most common medically confirmed adverse events were consistent with the known safety profile of levothyroxine, with generally comparable rates between both formulations (range of differences, 1.8-4.1%). Most cases were not serious (old formulation, 65.8%; new formulation, 78.7%). Reporting rates were similar or higher for the old formulation within subgroups of at-risk patients. Nature/distributions of adverse events by thyroid-stimulating hormone levels as determined by both the marketing authorization holder of levothyroxine and the French National Agency for Medicines and Health Products Safety were similar.

CONCLUSIONS

The new formulation safety profile aligns with the established profile of the old formulation of levothyroxine. The benefit-risk profile is unchanged, such that the benefits of using the new formulation in the approved indications outweigh the risks associated with the treatment.

Levothyroxine Formulations: Pharmacological and Clinical Implications of Generic Substitution

Oral levothyroxine (LT4) is the standard therapy for patients with hypothyroidism. Oral LT4 is available in several formulations, including tablets, soft gel capsules and oral solution. Multiple brand-name and generic LT4 tablets are available. In the US, the Food and Drug Administration (FDA) has developed a protocol for establishing bioequivalence of LT4 formulations based on serum thyroxine (T4) levels after a single oral dose administered to healthy volunteers. This protocol has been criticized by professional endocrinology associations for using healthy individuals and ignoring serum thyroid-stimulating hormone (TSH) levels. In addition, the protocol did not initially correct for baseline T4 levels, although this was changed in a later version. There are concerns that the FDA's protocol could allow products with clinically significant differences in bioavailability to be declared therapeutically equivalent and interchangeable. Once a generic LT4 has been shown to be bioequivalent to a brand-name LT4, it may be substituted for that brand-name LT4 with no need for dose adjustment or follow-up therapeutic monitoring. Often, the substitution is made by the pharmacy without the physician's knowledge. Even small differences between LT4 formulations can cause significant changes in TSH levels. This may be a particular concern in vulnerable populations, including elderly, pregnant, and pediatric patients. Problems that can be encountered when switching between formulations or when original products are reformulated are discussed in this review. These problems include altered efficacy and adverse events, some of which can be caused by excipients. Patients should be maintained on the same LT4 preparation if possible. If the LT4 preparation is changed, TSH levels should be evaluated and, if necessary, the dose of LT4 adjusted.Funding: Merck.Plain Language Summary: Plain language summary available for this article.

A new formulation of levothyroxine engineered to meet new specification standards

Background: Small variations in the dose of levothyroxine have been associated with marked variations in thyroid function in people with hypothyroidism. Accordingly, regulators have identified levothyroxine as a "narrow therapeutic index" drug subject to more stringent regulations compared with other drugs, in terms of the accuracy and stability of the amount of active drug in each tablet (typically required to be 95-105% of the labelled amount over its full shelf life), and its bioavailability geometric mean ratios (90% confidence intervals between 90-111.1%, including 100%).Review: This review describes a reformulation of a widely used levothyroxine product (Euthyrox.*). The new tablet fulfils all criteria according to the new specification regulations for dosage accuracy over a shelf life of 3 years in all climate zones, and for bioequivalence compared to the conventional formulation used for many years. In addition, a clinical trial demonstrated equivalent exposure between three different tablet strengths of the new formulation, amounting to the same total dose (dose form proportionality). As a consequence, switching from the conventional to the new formulation can be undertaken on a 1:1 dose-for-dose basis, without re-titration or additional thyroid function testing.Conclusion: The new formulation, which is more stable, will assist in the accurate dosage and titration of levothyroxine in the management of hypothyroidism.

New levothyroxine formulation meeting 95-105% specification over the whole shelf-life: results from two pharmacokinetic trials

OBJECTIVE

Small levothyroxine (L-T4) dose changes can lead to significant clinical effects. To ensure thyroid hormone levels are safely maintained, authorities are increasingly adopting stricter potency specifications for L-T4, the most stringent of these being 95-105% of the labeled dose over the whole shelf-life. Levothyroxine sodium (Euthyrox, Eutirox, Lévothyrox ) has been reformulated, and two studies performed, to ensure bioequivalence to the currently marketed formulation and dosage form proportionality of the new formulation.

METHODS

The bioequivalence study was an open-label, randomized, single-dose, two-period, two-sequence crossover comparing the highest dosage strengths of the currently marketed and the new L-T4 formulation at a total dose of 600 μg. The dosage form proportionality study was an open-label, randomized, three-period, six-sequence crossover, comparing 50 μg, 100 μg, and 200 μg L-T4 tablets, at a total dose of 600 μg. Blood samples were taken at predefined time intervals. Primary outcomes were area under the curve (AUC) and maximum concentration (C_max) of thyroxine (T4) in plasma.

RESULTS

In the bioequivalence study, comparing the T4 profiles for the new and current formulation of L-T4, the geometric least square mean ratio of the baseline-adjusted AUC_0-72,adj was 99.3% (90% confidence interval [CI]: 95.6-103.2) and the C_max,adj was 101.7% (90% CI: 98.8-104.6). Bioequivalence was established if the 90% CI lay within the predefined 0.9-1.11 limits. In the dosage form proportionality study, pairwise comparisons ranged from 99.3% to 104.8%, and all 95% CIs were within the predefined CI range (0.8-1.25): the three dose strengths were dosage form proportional.

CONCLUSIONS

The new formulation of L-T4 meets the most stringent potency specification guidelines, and has been demonstrated to be bioequivalent to the current formulation and to show dosage form proportionality. The new formulation will enable patients to receive a dose fine tuned to their medical needs, contributing to improved safety in the use of L-T4.

Early detection of biochemically occult autonomous thyroid nodules

OBJECTIVE

Autonomously functioning thyroid areas may be associated with subclinical or overt hyperthyroidism, but may exist even in the presence of normal TSH. This study was aimed at comparing the rate of autonomously functioning areas and their cardiac sequelae in patients with nodular goitre studied with the usual and a novel approach.

DESIGN AND METHODS

In total 490 adult outpatients with thyroid nodular goitre, living in a mild iodine-deficient area, were selected in our referral centre for thyroid diseases from 2009 to 2014 on the basis of a suspicion of thyroid functional autonomy. They were divided in three groups according to a non-conventional approach (excessive response to thyroxine treatment: group 1) or conventional approach (low/normal TSH with clinical suspicion or low TSH: groups 2 and 3). All patients of the study with the suspicion of thyroid functional autonomy underwent thyroid scan with radioactive iodine (I¹³¹) uptake (RAIU).

RESULTS

The percentage of confirmed thyroid functional autonomy was 319/490, being significantly higher in group 3 than in groups 1 and 2 (81.5 vs 64.7 vs 52.6%; chi-square P < 0.0001). However, the diagnosis with non-conventional approach was made at a significant earlier age (P < 0.0001). Cardiac arrhythmias as well as atrial fibrillation were similarly detected by conventional and non-conventional approaches (chi-square test: P = 0.2537; P = 0.8425).

CONCLUSIONS

The hyper-responsiveness to thyroxine treatment should induce the suspicion of thyroid functional autonomy at an early stage, allowing to detect autonomous functioning areas in apparently euthyroid patients.

Controlled Delivery of Levothyroxine Using Porous Silicon as a Drug Nanocontainer

Porous silicon (pSi) materials are regarded as promising drug delivery vehicles due to their biocompatibility, in vivo degradation, and resorption. We examine pSi films as a platform for the controlled delivery of levothyroxine, as a means to overcome problems with consistent dosage of this drug by oral administration. Oxidized pSi films were modified with 3-(aminopropyl)triethoxysilane (APTES), creating a surface chemistry that increased levothyroxine drug loading capacity by 50 % and sustained drug release under physiological conditions for 14 days. Release kinetics from APTES-functionalized films initially followed a zero-order release profile, which is highly desirable for drug delivery. The loading and release profiles of levothyroxine suggest that the film size required to deliver a therapeutic dose is feasible for further consideration as an implantable delivery system.

Generic levothyroxine compared with synthroid in young children with congenital hypothyroidism

CONTEXT

Clinicians who prescribe levothyroxine (LT4) for hypothyroidism often feel strongly about using a brand-name drug instead of a generic.

OBJECTIVE

The objective of the study was to determine whether Synthroid resulted in better control of congenital hypothyroidism than generic LT4.

DESIGN

This was a 5-year retrospective study.

SETTING

The study was conducted at 1 tertiary care center.

PATIENTS

Children who were 0-36 months old with congenital hypothyroidism followed up at our center from 2006 to 2011 were treated with either Synthroid exclusively (35 subjects) or generic LT4 exclusively (27 subjects).

INTERVENTIONS

We recorded the subjects' TSH and free T(4) measurements, how often their LT4 dose was adjusted, and the duration of follow-up.

MAIN OUTCOME MEASURE

TSH variance between the groups was measured. Secondary end points were the frequency of LT4 dose changes and the variance in free T(4).

RESULTS

Using the Wilcoxon rank sum test, there was no difference in TSH SD in the Synthroid group compared with the generic group (median 3.0 vs 2.2, P = .27). Using a linear mixed model, children treated with the generic LT4 had lower TSH estimated SD [1.35 with 95% confidence interval (CI) (1.194, 1.526)] than the Synthroid group [1.66 with 95% CI (1.536, 1.803)]. Similarly, no difference was observed in free T(4) SD between the groups using the Wilcoxon rank sum test (median 0.29 generic vs 0.36 Synthroid, P = .11), but the generic group had lower free T(4) estimated SD than the Synthroid group using the linear mixed model [0.216 with 95% CI (0.187, 0.249) vs 0.298 with 95% CI (0.273,0.326)]. Frequency of LT4 dosing adjustments was similar between the groups, both in total (median 2.0 for generic vs 3.0 for Synthroid, P = .097) and when adjusted for number of TSH checks (ratio 0.25 generic vs 0.31 Synthroid, P = .45).

CONCLUSIONS

In our study of congenital hypothyroidism, generic LT4 treatment resulted in similar or better control of hypothyroidism compared with Synthroid, as assessed by the clinical outcomes of TSH variance and the frequency of LT4 dosing adjustments.

An investigation into the influence of experimental conditions on in vitro drug release from immediate-release tablets of levothyroxine sodium and its relation to oral bioavailability

The aim of this study was to investigate the influence of experimental conditions on levothyroxine sodium release from two immediate-release tablet formulations which narrowly passed the standard requirements for bioequivalence studies. The in vivo study was conducted as randomised, single-dose, two-way cross-over pharmacokinetic study in 24 healthy subjects. The in vitro study was performed using various dissolution media, and obtained dissolution profiles were compared using the similarity factor value. Drug solubility in different media was also determined. The in vivo results showed narrowly passing bioequivalence. Considering that levothyroxine sodium is classified as Class III drug according to the Biopharmaceutics Classification System, drug bioavailability will be less sensitive to the variation in its dissolution characteristics and it can be assumed that the differences observed in vitro in some of investigated media probably do not have significant influence on the absorption process, as long as rapid and complete dissolution exists. The study results indicate that the current regulatory criteria for the value of similarity factor in comparative dissolution testing, as well as request for very rapid dissolution (more than 85% of drug dissolved in 15 min), are very restricted for immediate-release dosage forms containing highly soluble drug substance and need further investigation. The obtained results also add to the existing debate on the appropriateness of the current bioequivalence standards for levothyroxine sodium products.

Healthcare costs associated with switching from brand to generic levothyroxine

BACKGROUND

Controversy exists over the true therapeutic equivalence of branded and generic levothyroxine-the drug of choice for treating hypothyroidism-so professional societies recommend against switching between different formulations of the drug and suggest that patients who do switch be monitored. Payers typically encourage switching to generic drugs because of lower drug acquisition costs.

OBJECTIVE

To evaluate the impact of switching levothyroxine formulations on actual healthcare costs.

METHODS

Patients with hypothyroidism and at least 6 months of branded levothyroxine therapy were identified from a large healthcare claims database. Patients who subsequently switched to another levothyroxine formulation and could be followed for 6 months postswitch were matched to demographically similar patients who were continuous users of branded levothyroxine. Pre- and postswitch healthcare costs for each group were compared.

RESULTS

The savings in prescription drug costs after switching from branded to generic levothyroxine are offset by increases in costs for other healthcare services, such that switching is actually associated with an increase, not a decrease, in total healthcare costs.

CONCLUSION

In the absence of cost-savings, there is no clear rationale for switching patients from brand to generic levothyroxine.

TSH-based protocol, tablet instability, and absorption effects on L-T4 bioequivalence

BACKGROUND

FDA Guidance for pharmacokinetic (PK) testing of levothyroxine (L-T(4)) for interbrand bioequivalence has evolved recently. Concerns remain about efficacy and safety of the current protocol, based on PK analysis following supraphysiological L-T(4) dosing in euthyroid volunteers, and recent recalls due to intrabrand manufacturing problems also suggest need for further refinement. We examine these interrelated issues quantitatively, using simulated what-if scenarios testing efficacy of a TSH-based protocol and tablet stability and absorption, to enhance precision of L-T(4) bioequivalence methods.

METHODS

We use an updated simulation model of human thyroid hormone regulation quantified and validated from data that span a wide range of normal and abnormal thyroid system function. Bioequivalence: We explored a TSH-based protocol, using normal replacement dosing in simulated thyroidectomized patients, switching brands after 8 weeks of full replacement dosing. We simulated effects of tablet potency differences and intestinal absorption differences on predicted plasma TSH, T(4), and triiodothyronine (T(3)) dynamics. Stability: We simulated effects of potency decay and lot-by-lot differences in realistic scenarios, using actual tablet potency data spanning 2 years, comparing the recently reduced 95-105% FDA-approved potency range with the original 90-110% range.

RESULTS

A simulated decrease as small as 10-15% in L-T(4) or its absorption generated TSH concentrations outside the bioequivalence target range (0.5-2.5 mU/L TSH), whereas T(3) and T(4) plasma levels were maintained normal. For a 25% reduction, steady-state TSH changed 300% (from 1.5 to 6 mU/L) compared with <25% for both T(4) and T(3) (both within their reference ranges). Stability: TSH, T(4), and T(3) remained within normal ranges for most potency decay scenarios, but tablets of the same dose strength and brand were not bioequivalent between lots and between fresh and near-expired tablets.

CONCLUSIONS

A pharmacodynamic TSH-measurement bioequivalence protocol, using normal L-T(4) replacement dosing in athyreotic volunteers, is likely to be more sensitive and safer than current FDA Guidance based on T(4) PK. The tightened 95-105% allowable potency range for L-T(4) tablets is a significant improvement, but otherwise acceptable potency differences (whether due to potency decay or lot-by-lot inconsistencies) may be problematic for some patients, for example, those undergoing high-dose L-T(4) therapy for cancer.

Extensions, validation, and clinical applications of a feedback control system simulator of the hypothalamo-pituitary-thyroid axis

BACKGROUND

We upgraded our recent feedback control system (FBCS) simulation model of human thyroid hormone (TH) regulation to include explicit representation of hypothalamic and pituitary dynamics, and updated TH distribution and elimination (D&E) parameters. This new model greatly expands the range of clinical and basic science scenarios explorable by computer simulation.

METHODS

We quantified the model from pharmacokinetic (PK) and physiological human data and validated it comparatively against several independent clinical data sets. We then explored three contemporary clinical issues with the new model: combined triiodothyronine (T(3))/thyroxine (T(4)) versus T(4)-only treatment, parenteral levothyroxine (L-T(4)) administration, and central hypothyroidism.

RESULTS

Combined T(3)/T(4) therapy--In thyroidectomized patients, the L-T(4)-only replacement doses needed to normalize plasma T(3) or average tissue T(3) were 145 microg L-T(4)/day or 165 microg L-T(4)/day, respectively. The combined T(4) + T(3) dosing needed to normalize both plasma and tissue T(3) levels was 105 microg L-T(4) + 9 microg T(3) per day. For all three regimens, simulated mean steady-state plasma thyroid-stimulating hormone (TSH), T(3), and T(4) was within normal ranges (TSH: 0.5-5 mU/L; T(4): 5-12 microg/dL; T(3): 0.8-1.9 ng/mL). Parenteral T(4) administration--800 microg weekly or 400 microg twice weekly normalized average tissue T(3) levels both for subcutaneous (SC) and intramuscular (IM) routes of administration. TSH, T(3), and T(4) levels were maintained within normal ranges for all four of these dosing schemes (1x vs. 2x weekly, SC vs. IM). Central hypothyroidism--We simulated steady-state plasma T(3), T(4), and TSH concentrations in response to varying degrees of central hypothyroidism, reducing TSH secretion from 50% down to 0.1% of normal. Surprisingly, TSH, T(3), and T(4) plasma concentrations remained within normal ranges for TSH secretion as low as 25% of normal.

CONCLUSIONS

Combined T(3)/T(4) treatment--Simulated standard L-T(4)-only therapy was sufficient to renormalize average tissue T(3) levels and maintain normal TSH, T(3), and T(4) plasma levels, supporting adequacy of standard L-T(4)-only treatment. Parenteral T(4) administration-TSH, T(3), and T(4) levels were maintained within normal ranges for all four of these dosing schemes (1x vs. 2x weekly, SC vs. IM), supporting these therapeutic alternatives for patients with compromised L-T(4) gut absorption. Central hypothyroidism--These results highlight how highly nonlinear feedback in the hypothalamic-pituitary-thyroid axis acts to maintain normal hormone levels, even with severely reduced TSH secretion.

UGT1A1*28 genotype affects the in-vitro glucuronidation of thyroxine in human livers

OBJECTIVE

L-thyroxine (T4), the most widely used drug for hypothyroidism, undergoes glucuronidation by UDP-glucuronosyltransferases. Clinical evidence obtained after the administration of anticonvulsants suggest that glucuronidation may play an important role in T4 homeostasis in humans. The aims of this study were to determine the T4 glucuronidation ability of all commercially available human UGTs, and investigate the relationship between genetic polymorphisms in UGT1A1 and UGT1A9 and T4 glucuronidation in human livers.

METHODS

Glucuronidation of T4 in human liver microsomes and recombinant UDP-glucuronosyltransferases was measured by high-pressure liquid chromatography. UGT1A1 -53(TA)6>7 (UGT1A1*28) and UGT1A9 -118T9>10 (UGT1A9*1b) variants were genotyped by polymerase chain reaction and sizing.

RESULTS

A strong correlation was observed between the glucuronidation of T4 and SN-38, a UGT1A1 substrate (r=0.82, P<0.0001). A significant trend of decreasing T4 glucuronide (T4G) levels was observed with increasing number of UGT1A1 -53(TA)7 alleles (P=0.001). Other hepatic UDP-glucuronosyltransferases involved in T4G formation are UGT1A3 and UGT1A9. No significant relationship was observed between UGT1A9 -118T9>10 and T4 glucuronidation activity. T4 can also undergo glucuronidation by UGT1A8 and UGT1A10, which are expressed in the gastrointestinal tract (but not the liver) and may be important for first-pass T4 metabolism.