Foslevodopa/Foscarbidopa: A New Subcutaneous Treatment for Parkinson's Disease.

OBJECTIVE: The aim was to demonstrate that continuous s.c. infusion of a soluble levodopa (LD)/carbidopa (CD) phosphate prodrug combination effectively delivers stable LD exposure via a minimally invasive and convenient mode and has the potential to treat Parkinson's disease (PD) patients who are not well controlled on oral medication. METHODS: Foslevodopa and foscarbidopa were prepared and the equilibrium solubility and chemical stability examined in aqueous media with different values of pH. Solutions of foslevodopa/foscarbidopa (ratios ranging from 4:1 to 20:1) were prepared by dissolving pH-adjusted lyophilized materials in water and infused s.c. in healthy volunteers for ≤72 hours. Frequent blood samples were collected to measure LD and CD exposure, and safety was monitored throughout the study. RESULTS: Foslevodopa/foscarbidopa (ABBV-951) demonstrates high water solubility and excellent chemical stability near physiological pH, enabling continuous s.c. infusion therapy. After s.c. infusion, a stable LD pharmacokinetic (PK) profile was maintained for ≤72 hours, and the infusion was well tolerated. INTERPRETATION: Preparation of foslevodopa and foscarbidopa enables preclinical and clinical PK, safety, and tolerability studies in support of their advancement for the treatment of PD. In phase 1 clinical trials, foslevodopa/foscarbidopa demonstrates consistent and stable LD plasma exposure, supporting further studies of this treatment as a potentially transformational option for those suffering from PD. ANN NEUROL 2021;90:52-61.

Overcoming Bioavailability Challenges of Dasabuvir and Enabling a Triple-Combination Direct-Acting Antiviral HCV Regimen through a Salt of Very Weak Acid for Oral Delivery.

Dasabuvir is a non-nucleoside polymerase inhibitor for the treatment of hepatitis C virus (HCV) infection. It is an extremely weak diacidic drug (pKa = 8.2 and 9.2) and a prolific solvate former. Due to its exceedingly low aqueous solubility (≤0.127 µg/mL at pH 1-6.8, dose number of 1.31 × 104), crystalline dasabuvir free acid exhibited poor oral bioavailability in initial animal pharmacokinetic (PK) assessment. This necessitated the development of enabling formulation for human clinical studies to achieve the required therapeutic in vivo concentration of dasabuvir. While salt formation has been widely used to enhance the solubility and dissolution rate of solids, this approach has rarely been applied to develop oral solid dosage forms for acidic drugs as weak as dasabuvir due to concerns of rapid disproportionation and crystallization of its free acid. In this contribution, we detail our efforts in identifying dasabuvir monosodium monohydrate as a drug substance that is stable, manufacturable, and, most importantly, significantly enhances the dissolution and oral absorption of this poorly soluble drug. The oral delivery of dasabuvir through the salt approach has enabled the commercialization of the triple-combination direct-acting antiviral HCV regimen, Viekira Pak. The methodologies and solutions identified in targeted studies to overcome technical challenges encountered along the way (i.e., incorporation of polymers to inhibit crystallization and disproportionation and species mapping to enable salt manufacturing process, etc.) can be applied to other insoluble compounds.

Discovery of 2-aminoisobutyric acid ethyl ester (AIBEE) phosphoramidate prodrugs for delivering nucleoside HCV NS5B polymerase inhibitors.

Our HCV research program investigated novel 2'-dihalogenated nucleoside HCV polymerase inhibitors and identified compound 1, a 5'-phosphoramidate prodrug of 2'-deoxy-2'-α-bromo-ß-chloro uridine. Although 1 had a favorable in vitro activity profile in HCV replicons, oral dosing in dog resulted in low levels of the active 5'-triphosphate (TP) in liver. Metabolism studies using human hepatocytes provided a simple assay for screening alternative phosphoramidate prodrug analogs. Compounds that produced high TP concentrations in hepatocytes were tested in dog liver biopsy studies. This method identified 2-aminoisobutyric acid ethyl ester (AIBEE) phosphoramidate prodrug 14, which provided 100-fold higher TP concentrations in dog liver in comparison to 1 (4 and 24 h after 5 mg/kg oral dose).

Synthesis and evaluation of 2'-dihalo ribonucleotide prodrugs with activity against hepatitis C virus.

Hepatitis C virus (HCV) nucleoside inhibitors have been a key focus of nearly 2 decades of HCV drug research due to a high barrier to drug resistance and pan-genotypic activity profile provided by molecules in this drug class. Our investigations focused on several potent 2'-halogenated uridine-based HCV polymerase inhibitors, resulting in the discovery of novel 2'-deoxy-2'-dihalo-uridine analogs that are potent inhibitors in replicon assays for all genotypes. Further studies to improve in vivo performance of these nucleoside inhibitors identified aminoisobutyric acid ethyl ester (AIBEE) phosphoramidate prodrugs 18a and 18c, which provide high levels of the active triphosphate in dog liver. AIBEE prodrug 18c was compared with sofosbuvir (1) by co-dosing both compounds by oral administration in dog (5 mg/kg each) and measuring liver concentrations of the active triphosphate metabolite at both 4 and 24 h post dosing. In this study, 18c provided liver triphosphate concentrations that were 6-fold higher than sofosbuvir (1) at both biopsy time points, suggesting that 18c could be a highly effective agent for treating HCV infected patients in the clinic.

Assessing the Impact of Endogenously Derived Crystalline Drug on the in Vivo Performance of Amorphous Formulations.

Crystallization of drug from an amorphous formulation is expected to negatively impact its bioperformance following oral delivery. In evaluating this in vivo, neat crystalline drug is typically mixed with the amorphous formulation. However, this approach may not adequately mimic the effect of drug crystals that form within the amorphous matrix, because crystal properties are highly dependent on the crystallization environment. The aim of this study was to evaluate the in vivo impact of crystals formed in a generic tacrolimus amorphous formulation, relative to noncrystallized formulations and a reference suspension containing neat crystalline drug. Crystallization of tacrolimus was induced in the generic product by exposing it to moderate temperatures and high relative humidity. Controlled levels of crystallinity in the formulations were achieved by mixing maximally crystallized and fresh formulations at the desired ratios. These formulations were then characterized in vitro and used for oral dosing to beagle dogs. Analysis of blood concentrations versus time revealed that formulations containing 50 and 100% crystalline tacrolimus resulted in lower area under the curve (AUC) and maximum concentration (Cmax) values as compared to the fresh amorphous formulation. However, the AUC and the Cmax values for these formulations were significantly higher than those observed after dosing the pure crystalline tacrolimus suspension. The innovator formulation, Prograf, showed comparable pharmacokinetics before and after exposure to accelerated stability conditions, confirming the robustness of the innovator product to drug crystallization. This study provides insight into the impact of endogenously crystallized material on the oral absorption of a poorly water-soluble compound and highlights the importance of using representative crystalline material when undertaking risk assessment of amorphous formulations.

Discovery of fluorobenzimidazole HCV NS5A inhibitors.

Research toward a next-generation HCV NS5A inhibitor has identified fluorobenzimidazole analogs that demonstrate potent, broad-genotype in vitro activity against HCV genotypes 1-6 replicons as well as HCV NS5A variants that are orders of magnitude less susceptible to inhibition by first-generation NS5A inhibitors in comparison to wild-type replicons. The fluorobenzimidazole inhibitors have improved pharmacokinetic properties in comparison to non-fluorinated benzimidazole analogs. Discovery of these inhibitors was facilitated by exploring SAR in a structurally simplified inhibitor series.

Discovery of a Potent Chloroacetamide GPX4 Inhibitor with Bioavailability to Enable Target Engagement in Mice, a Potential Tool Compound for Inducing Ferroptosis In Vivo.

Compounds that inhibit glutathione peroxidase 4 (GPX4) hold promise as cancer therapeutics in their ability to induce a form of nonapoptotic cell death called ferroptosis. Our research identified 24, a structural analog of the potent GPX4 inhibitor RSL3, that has much better plasma stability (t1/2 > 5 h in mouse plasma). The bioavailability of 24 provided efficacious plasma drug concentrations with IP dosing, thus enabling in vivo studies to assess tolerability and efficacy. An efficacy study in mouse using a GPX4-sensitive tumor model found that doses of 24 up to 50 mg/kg were tolerated for 20 days but had no effect on tumor growth, although partial target engagement was observed in tumor homogenate.

Expanding the Repertoire for "Large Small Molecules": Prodrug ABBV-167 Efficiently Converts to Venetoclax with Reduced Food Effect in Healthy Volunteers.

Since gaining approval for the treatment of chronic lymphocytic leukemia (CLL), the BCL-2 inhibitor venetoclax has transformed the treatment of this and other blood-related cancers. Reflecting the large and hydrophobic BH3-binding groove within BCL-2, venetoclax has significantly higher molecular weight and lipophilicity than most orally administered drugs, along with negligible water solubility. Although a technology-enabled formulation successfully achieves oral absorption in humans, venetoclax tablets have limited drug loading and therefore can present a substantial pill burden for patients in high-dose indications. We therefore generated a phosphate prodrug (3, ABBV-167) that confers significantly increased water solubility to venetoclax and, upon oral administration to healthy volunteers either as a solution or high drug-load immediate release tablet, extensively converts to the parent drug. Additionally, ABBV-167 demonstrated a lower food effect with respect to venetoclax tablets. These data indicate that beyond-rule-of-5 molecules can be successfully delivered to humans via a solubility-enhancing prodrug moiety to afford robust exposures of the parent drug following oral dosing.

Amorphous solid dispersions of enzalutamide and novel polysaccharide derivatives: investigation of relationships between polymer structure and performance.

Amorphous solid dispersion (ASD) is a widely employed formulation technique for drugs with poor aqueous solubility. Polymers are integral components of ASDs, but mechanisms by which polymers lead to the generation and maintenance of supersaturated solutions, which enhance oral absorption in vivo, are poorly understood. Herein, a diverse group of newly synthesized cellulose derivatives was evaluated for their ability to inhibit crystallization of enzalutamide, a poorly soluble compound used to treat prostate cancer. ASDs were prepared from selected polymers, specifically a somewhat hydrophobic polymer that was extremely effective at inhibiting drug crystallization, and a less effective, but more hydrophilic, crystallization inhibitor, that might afford better release. Drug membrane transport rate was evaluated in vitro and compared to in vivo performance, following oral dosing in rats. Good correlation was noted between the in vitro diffusion cell studies and the in vivo data. The ASD formulated with the less effective crystallization inhibitor outperformed the ASD prepared with the highly effective crystallization inhibitor in terms of the amount and rate of drug absorbed in vivo. This study provides valuable insight into key factors impacting oral absorption from enabling ASD formulations, and how best to evaluate such formulations using in vitro approaches.

Prodrug Strategies to Improve the Solubility of the HCV NS5A Inhibitor Pibrentasvir (ABT-530).

A research program to discover solubilizing prodrugs of the HCV NS5A inhibitor pibrentasvir (PIB) identified phosphomethyl analog 2 and trimethyl-lock (TML) prodrug 9. The prodrug moiety is attached to a benzimidazole nitrogen atom via an oxymethyl linkage to allow for rapid and complete release of the drug for absorption following phosphate removal by intestinal alkaline phosphatase. These prodrugs have good hydrolytic stability properties and improved solubility compared to PIB, both in aqueous buffer (pH 7) and FESSIF (pH 5). TML prodrug 9 provided superior in vivo performance, delivering high plasma concentrations of PIB in PK studies conducted in mice, dogs, and monkeys. The improved dissolution properties of these phosphate prodrugs provide them the potential to simplify drug dosage forms for PIB-containing HCV therapy.

Discovery of ABBV-4083, a novel analog of Tylosin A that has potent anti-Wolbachia and anti-filarial activity.

There is a significant need for improved treatments for onchocerciasis and lymphatic filariasis, diseases caused by filarial worm infection. In particular, an agent able to selectively kill adult worms (macrofilaricide) would be expected to substantially augment the benefits of mass drug administration (MDA) with current microfilaricides, and to provide a solution to treatment of onchocerciasis / loiasis co-infection, where MDA is restricted. We have identified a novel macrofilaricidal agent, Tylosin A (TylA), which acts by targeting the worm-symbiont Wolbachia bacterium. Chemical modification of TylA leads to improvements in anti-Wolbachia activity and oral pharmacokinetic properties; an optimized analog (ABBV-4083) has been selected for clinical evaluation.

Relationship between amorphous solid dispersion in vivo absorption and in vitro dissolution: phase behavior during dissolution, speciation, and membrane mass transport.

Enzalutamide is a fast crystallizing, hydrophobic compound that has solubility limited absorption in vivo. Given the low aqueous solubility of this compound, it was of interest to evaluate amorphous formulations in vitro and in vivo. Amorphous solid dispersions (ASD) of enzalutamide were prepared with the hydrophilic polymers, hydroxypropyl methylcellulose acetate succinate (HPMCAS) and copovidone (PVPVA). A side-by-side diffusion cell was developed as an in vitro characterization tool to discriminate enzalutamide ASDs based upon the solute thermodynamic activity achieved during dissolution and its impact on the subsequent membrane transport rates, phase behavior, and drug speciation. The same formulations were then tested in vivo in rats using oral dosing of ASD suspensions. Different levels of plasma exposure were observed between the ASDs, which could be correlated to the phase behaviors of the ASDs following dissolution. Unsurprisingly, ASDs that underwent crystallization show lower plasma exposures. However, differences were also observed between ASDs that dissolved to form nanosized amorphous drug aggregates versus those that dissolved to yield only supersaturated solutions, with the former outperforming the latter in terms of the plasma exposure. These observations highlight the importance of thoroughly understanding the phase behavior of an amorphous formulation following dissolution and the need to discriminate between different types of precipitation, specifically crystallization versus glass liquid phase separation to form nanosized amorphous aggregates.

From the Cover: Inhibitors of Nicotinamide Phosphoribosyltransferase Cause Retinal Damage in Larval Zebrafish.

Nicotinamide phosphoribosyltransferase (NAMPT) has been investigated as a target for oncology because it catalyzes a rate-limiting step in cellular energy metabolism to produce nicotinamide adenine dinucleotide. Small molecule inhibitors of NAMPT have been promising drug candidates but preclinical development has been hindered due to associated retinal toxicity. Here we demonstrate that larval zebrafish can predict retinal toxicity associated with this mechanism revealing an attractive alternative method for identifying such toxicities. Zebrafish permit higher throughput testing while using far lower quantities of test article compared with mammalian systems. NAMPT inhibitor-associated toxicity manifested in zebrafish as a loss of response to visual cues compared with auditory cues. Zebrafish retinal damage associated with NAMPT inhibitor treatment was confirmed through histopathology. Ranking 6 NAMPT inhibitors according to their impact on zebrafish vision revealed a positive correlation with their in vitro potencies on human tumor cells. This correlation indicates translatable pharmacodynamics between zebrafish and human NAMPT and is consistent with on-target activity as the cause of retinal toxicity associated with NAMPT inhibition. Together, these data illustrate the utility of zebrafish for identifying compounds that may cause ocular toxicity in mammals, and, likewise, for accelerating development of compounds with improved safety margins.

Highlights of the Structure-Activity Relationships of Benzimidazole Linked Pyrrolidines Leading to the Discovery of the Hepatitis C Virus NS5A Inhibitor Pibrentasvir (ABT-530).

Curative interferon and ribavirin sparing treatments for hepatitis C virus (HCV)-infected patients require a combination of mechanistically orthogonal direct acting antivirals. A shared component of these treatments is usually an HCV NS5A inhibitor. First generation FDA approved treatments, including the component NS5A inhibitors, do not exhibit equivalent efficacy against HCV virus genotypes 1-6. In particular, these first generation NS5A inhibitors tend to select for viral drug resistance. Ombitasvir is a first generation HCV NS5A inhibitor included as a key component of Viekira Pak for the treatment of patients with HCV genotype 1 infection. Since the launch of next generation HCV treatments, functional cure for genotype 1-6 HCV infections has been achieved, as well as shortened treatment duration across a wider spectrum of genotypes. In this paper, we show how we have modified the anchor, linker, and end-cap architecture of our NS5A inhibitor design template to discover a next generation NS5A inhibitor pibrentasvir (ABT-530), which exhibits potent inhibition of the replication of wild-type genotype 1-6 HCV replicons, as well as improved activity against replicon variants demonstrating resistance against first generation NS5A inhibitors.

Discovery and metabolic stabilization of potent and selective 2-amino-N-(adamant-2-yl) acetamide 11beta-hydroxysteroid dehydrogenase type 1 inhibitors.

Starting from a rapidly metabolized adamantane 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) inhibitor 22a, a series of E-5-hydroxy-2-adamantamine inhibitors, exemplified by 22d and (+/-)-22f, was discovered. Many of these compounds are potent inhibitors of 11beta-HSD1 and are selective over 11beta-HSD2 for multiple species (human, mouse, and rat), unlike other reported species-selective series. These compounds have good cellular potency and improved microsomal stability. Pharmacokinetic profiling in rodents indicated moderate to large volumes of distribution, short half-lives, and a pharmacokinetic species difference with the greatest exposure measured in rat with 22d. One hour postdose liver, adipose, and brain tissue 11beta-HSD1 inhibition was confirmed with (+/-)-22f in a murine ex vivo assay. Although 5,7-disubstitued-2-adamantamines provided greater stability, a single, E-5-position, polar functional group afforded inhibitors with the best combination of stability, potency, and selectivity. These results indicate that adamantane metabolic stabilization sufficient to obtain short-acting, potent, and selective 11beta-HSD1 inhibitors has been discovered.

Measuring drug absorption improves interpretation of behavioral responses in a larval zebrafish locomotor assay for predicting seizure liability.

INTRODUCTION: Unanticipated effects on the central nervous system are a concern during new drug development. A larval zebrafish locomotor assay can reveal seizure liability of experimental molecules before testing in mammals. Relative absorption of compounds by larvae is lacking in prior reports of such assays; having those data may be valuable for interpreting seizure liability assay performance. METHODS: Twenty-eight reference drugs were tested at multiple dose levels in fish water and analyzed by a blinded investigator. Responses of larval zebrafish were quantified during a 30min dosing period. Predictive metrics were calculated by comparing fish activity to mammalian seizure liability for each drug. Drug level analysis was performed to calculate concentrations in dose solutions and larvae. Fifteen drug candidates with neuronal targets, some having preclinical convulsion findings in mammals, were tested similarly. RESULTS: The assay has good predictive value of established mammalian responses for reference drugs. Analysis of drug absorption by larval fish revealed a positive correlation between hyperactive behavior and pro-convulsive drug absorption. False negative results were associated with significantly lower compound absorption compared to true negative, or true positive results. The predictive value for preclinical toxicology findings was inferior to that suggested by reference drugs. DISCUSSION: Disproportionately low exposures in larvae giving false negative results demonstrate that drug exposure analysis can help interpret results. Due to the rigorous testing commonly performed in preclinical toxicology, predicting convulsions in those studies may be more difficult than predicting effects from marketed drugs.

Mechanistic Dissection of PARP1 Trapping and the Impact on In Vivo Tolerability and Efficacy of PARP Inhibitors.

UNLABELLED: Poly(ADP-ribose) polymerases (PARP1, -2, and -3) play important roles in DNA damage repair. As such, a number of PARP inhibitors are undergoing clinical development as anticancer therapies, particularly in tumors with DNA repair deficits and in combination with DNA-damaging agents. Preclinical evidence indicates that PARP inhibitors potentiate the cytotoxicity of DNA alkylating agents. It has been proposed that a major mechanism underlying this activity is the allosteric trapping of PARP1 at DNA single-strand breaks during base excision repair; however, direct evidence of allostery has not been reported. Here the data reveal that veliparib, olaparib, niraparib, and talazoparib (BMN-673) potentiate the cytotoxicity of alkylating agents. Consistent with this, all four drugs possess PARP1 trapping activity. Using biochemical and cellular approaches, we directly probe the trapping mechanism for an allosteric component. These studies indicate that trapping is due to catalytic inhibition and not allostery. The potency of PARP inhibitors with respect to trapping and catalytic inhibition is linearly correlated in biochemical systems but is nonlinear in cells. High-content imaging of γH2Ax levels suggests that this is attributable to differential potentiation of DNA damage in cells. Trapping potency is inversely correlated with tolerability when PARP inhibitors are combined with temozolomide in mouse xenograft studies. As a result, PARP inhibitors with dramatically different trapping potencies elicit comparable in vivo efficacy at maximum tolerated doses. Finally, the impact of trapping on tolerability and efficacy is likely to be context specific. IMPLICATIONS: Understanding the context-specific relationships of trapping and catalytic inhibition with both tolerability and efficacy will aid in determining the suitability of a PARP inhibitor for inclusion in a particular clinical regimen.

Chemical renal denervation in the rat.

INTRODUCTION: The recent success of renal denervation in lowering blood pressure in drug-resistant hypertensive patients has stimulated interest in developing novel approaches to renal denervation including local drug/chemical delivery. The purpose of this study was to develop a rat model in which depletion of renal norepinephrine (NE) could be used to determine the efficacy of renal denervation after the delivery of a chemical to the periadventitial space of the renal artery. METHODS: Renal denervation was performed on a single renal artery of 90 rats (n = 6 rats/group). The first study determined the time course of renal denervation after surgical stripping of a renal artery plus the topical application of phenol in alcohol. The second study determined the efficacy of periadventitial delivery of hypertonic saline, guanethidine, and salicylic acid. The final study determined the dose-response relationship for paclitaxel. In all studies, renal NE content was determined by liquid chromatography-mass spectrometry. RESULTS: Renal NE was depleted 3 and 7 days after surgical denervation. Renal NE was also depleted by periadventitial delivery of all agents tested (hypertonic saline, salicylic acid, guanethidine, and paclitaxel). A dose response was observed after the application of 150 µL of 10(-5) M through 10(-2) M paclitaxel. CONCLUSION: We developed a rat model in which depletion of renal NE was used to determine the efficacy of renal denervation after perivascular renal artery drug/chemical delivery. We validated this model by demonstrating the efficacy of the neurotoxic agents hypertonic saline, salicylic acid, and guanethidine and increasing doses of paclitaxel.

Clinical efficacy of sustained-release buprenorphine with meloxicam for postoperative analgesia in beagle dogs undergoing ovariohysterectomy.

The goal of the current study was to compare the efficacy, adverse effects, and plasma buprenorphine concentrations of sustained-release buprenorphine (SRB) and buprenorphine after subcutaneous administration in dogs undergoing ovariohysterectomy. In a prospective, randomized, blinded design, 20 healthy adult female Beagle dogs underwent routine ovariohysterectomy and received multimodal analgesia consisting of meloxicam and one of two buprenorphine formulations. Dogs were randomly assigned to receive either SRB (0.2 mg/kg SC, once) or buprenorphine (0.02 mg/kg SC every 12 h for 3 d). Blinded observers assessed all dogs by using sedation scores, pain scores, temperature, HR, RR, and general wellbeing. Dogs were provided rescue analgesia with 0.02 mg/kg buprenorphine SC if the postoperative pain score exceeded a prede- termined threshold. Blood samples were collected, and mass spectrometry was used to determine plasma buprenorphine concentrations. Data were analyzed with a linear mixed model and Tukey-Kramer multiple comparison. Age, body weight, anesthetic duration, surgical duration, sevoflurane concentration, and cardiorespiratory variables did not differ significantly between groups. Dogs in both formulation groups had comparable postoperative sedation and pain scores. One dog from each formulation group had breakthrough pain requiring rescue analgesia. Plasma buprenorphine concentrations remained above a hypothesized therapeutic concentration of 0.6 ng/mL for 136.0 ± 11.3 and 10.67 ± 0.84 h for SRB and buprenorphine, respectively. Based on the results of this study, multimodal analgesic regimens consisting of meloxicam and either buprenorphine or SRB are equally efficacious in managing pain associated with an ovariohysterectomy and show comparable side effects.

Pharmacokinetics of sustained-release and transdermal buprenorphine in Göttingen minipigs (Sus scrofa domestica).

The opioid buprenorphine has been shown to provide adequate postoperative analgesia in both companion and laboratory animals. However, its use is still hindered by the need for multiple parenteral injections to achieve continuous analgesia. The purpose of the current study was to conduct a pharmacokinetic analysis of 2 new long-acting formulations of buprenorphine-an injectable sustained-release buprenorphine (SRB) and a transdermal buprenorphine (TDB) patch-in healthy Göttingen minipigs by using liquid chromatography-electrospray ionization-tandem mass spectrometry. Administration of 0.18 mg/kg SC SRB and 30 µ g/h TDB achieved AUC(0-Tlast) of 221.6 ± 26.8 and 25.2 ± 3.9 ng × h/mL, respectively, compared with 9.7 ± 1.4 ng*h/mL for 0.02 mg/kg IV buprenorphine. By using a hypothesized therapeutic plasma buprenorphine concentration threshold of 0.1 ng/mL, therapeutic concentrations were achieved at the first study time point (5 to 30 min) and lasted an average of 8.0 ± 1.3 h for intravenous buprenorphine and 264.0 ± 32.2 h for SRB. TDB achieved therapeutic concentrations in 12 to 24 h after patch application, which lasted until the patch was removed at 72 h. The results of this study suggest that SRB and TDB are long-acting alternatives for pain management, and their use could decrease animal handling and stress, thereby simplifying pain management and improving welfare in laboratory swine.