Characterization of platelet aggregation responses in microminipigs: Comparison with miniature pigs and the influence of dual antiplatelet administration of aspirin plus prasugrel

Comparative Evaluation of Two Paclitaxel-Coated Stents in an Experimental Setting

INTRODUCTION

Unlike paclitaxel-coated balloons, pre-clinical data comparing different paclitaxel-coated stents (PCSs) are weak. The study objective was to compare the features of the 2 main PCSs: Eluvia® (Boston Scientific, Marlborough, MA) versus ZilverPTX® (Cook Medical, Bloomington, IN).

METHOD

Analysis was carried out on 12 pigs divided into 2 groups: Eluvia® (n=6) and ZilverPTX® (n=6). The pigs received the PCS corresponding to their group in each external iliac artery and were paired one by one, to examine 6 different post-implantation timepoints: after 30 minutes, 6 hours, 24 hours, 3 days, 7 days, and 14 days. The paclitaxel concentration measurements and the histological analysis were carried out under blind testing on the plasma, arterial, lymph node, and muscle samples. A linear regression model and Wilcoxon Mann-Whitney test were used to study the variables.

RESULTS

The plasma paclitaxel rate decrease over 24 hours after PCS implantation was significantly different between the two groups, expressed by the correlation coefficient 0.19 (0.14-0.23; p<0.001) with an undetectable concentration at the 10th hour for Eluvia® versus 3 days for ZilverPTX®. Significantly higher paclitaxel concentrations with ZilverPTX® PCS were observed in muscle samples at each timepoint: extensor digitorum brevis 3.2 (1.17-5.23; p=0.005), biceps femoris 4.27 (2.27-6.26; p<0.001), semi-tendinosus 3.79 (1.85-5.73; p=0.001), tibialis anterior 3.0 (1.37-4.64; p=0.001), and in the femoral nodes 2.27±1.74 ng/g versus 0.14±0.13 ng/g (p<0.001). Histological analysis revealed a trend for more marked intimal inflammation in the arteries stented with ZilverPTX® (p=0.063), especially after the 7th and 14th days.

CONCLUSION

Such a difference in the concentration of paclitaxel in the plasma, muscles, and lymph nodes between the two stents was higher than expected based on differences in device design. The clinical consequences of these results remain to be elucidated, particularly regarding the concerning presence of paclitaxel in muscles and adjacent lymph nodes.

CLINICAL IMPACT

This experimental study compares 2 paclitaxel-coated stents. It demonstrates that differences in stent designs and drug features (coatings and concentrations) translate into differences in terms of concentrations of paclitaxel in the plasma, muscles, and lymph nodes. Our results favor the Eluvia® stent over the ZilverPTX® stent, although more studies are required to confirm this conclusion.

[Current progress of research and use of microminipigs in drug development]

The use of minipigs has been increasing in the areas of pharmacology researches and drug development. The microminipig developed by Fuji Micra Inc. (Shizuoka, Japan) inherits characteristics of other pig strains showing several similarities to humans in anatomy, physiology, omnivorousness and diurnal, but at the same time has several advantages over other pig strains because of its small size which allows easy keeping, handling and dosing, and saving of test substances. The microminipig weighs about 10 kg at the age of 6 months. Canine cages can be used to keep the animal. Swine leukocyte antigens (SLA) are defined in each individual animal which is useful for testing immunological reactions. As there are many similarities in metabolic enzymes and transporters to those in humans, the microminipig is a powerful animal model for toxicokinetic studies. Unfortunately as in other minipigs the microminipig is not appropriate for embryo-fetal development studies of antibody drugs due to its poor placental transfer, but can be used for other reproductive and developmental studies. Repeat dose toxicity, safety pharmacology, immunotoxicity and local tolerance studies should be also other arenas of this animal model.

A novel porcine model of thrombotic myocardial infarction with cardiac dysfunction sensitive to dual antiplatelet therapy

Few effective porcine models of myocardial infarction (MI) related to platelet thrombus formation are available. In this study, we established a novel porcine MI model and examined the effect of dual antiplatelet therapy (DAPT) with aspirin and prasugrel, a P2Y₁₂ antagonist, using this MI model. Thrombotic MI was photochemically induced using rose bengal. Male miniature pigs were divided into 3 treatment groups: Sham, MI, and DAPT. In the DAPT group, aspirin (10 mg/kg, p.o.) and prasugrel (1 mg/kg, p.o.) were administered 4 h before photo-irradiation. Platelet aggregation, MI volume, and cardiac function were evaluated 24 h after photo-irradiation. Inhibition of ADP-induced platelet aggregation in the DAPT group was about 45%, similar to the effects of DAPT in a clinical setting. No MI was observed in the Sham group, and MI volume was 12.9 ± 2.9% in the left ventricle (P = 0.0016) in the MI group. Additionally, an increase in end-systolic volume (P = 0.0006), and a decrease in stroke volume (P = 0.0001) and ejection fraction (P < 0.0001) were observed in the MI group compared to the Sham group without any changes in end-diastolic volume. DAPT significantly decreased MI volume (P = 0.0006) and ameliorated cardiac dysfunction compared to the MI group. In conclusion, a novel porcine model of thrombotic MI with cardiac dysfunction was established. In this model, DAPT decreased MI volume and ameliorated of cardiac dysfunction, suggesting that this porcine MI model could be useful for future research on MI and antithrombotic agents.

Effects of ticagrelor on neointimal hyperplasia and endothelial function, compared with clopidogrel and prasugrel, in a porcine coronary stent restenosis model

BACKGROUND

Several investigations have been conducted to evaluate the off-target effects of ticagrelor. The aim of the present study was to evaluate the off-target effects of ticagrelor such as neointimal formation and endothelial function after drug-eluting stent implantation in a porcine restenosis model.

METHODS

A total of 30 pigs were randomly allocated based on the following P2Y₁₂ inhibitor: (1) clopidogrel 300mg loading plus 75mg maintenance (n=10); (2) prasugrel 60mg loading plus 10mg maintenance (n=10); (3) ticagrelor 180mg loading plus 180mg maintenance (n=10). In each group, zotarolimus-eluting stents were implanted in the proximal portion of the left anterior descending artery and left circumflex artery. One month after stenting, the animals underwent follow-up angiography, endothelial function assessment, optical coherence tomography (OCT) and histopathological analysis.

RESULTS

Regarding vasomotor responses to acetylcholine infusion, there were significant vasoconstrictions to maximal acetylcholine infusion in the clopidogrel and prasugrel group compared with those in the ticagrelor group. The mean neointimal area were significantly lower in the ticagrelor group (1.0±0.3 by OCT, 0.9±0.3 by histology), than in the clopidogrel (1.8±0.7, p=0.003, 1.6±0.8, p=0.030) and prasugrel (1.8±0.5, p=0.001, 1.5±0.5, p=0.019) groups. Percentages of moderate to dense peri-strut inflammatory cell infiltration were significantly lower in the ticagrelor group (9.0%) compared with the clopidogrel (17.3%, p<0.001) and prasugrel groups (15.7%, p=0.002). There were no significant differences in all findings between clopidogrel and prasugrel groups.

CONCLUSIONS

Compared to clopidogrel and prasugrel, ticagrelor reduced neointimal formation, endothelial dysfunction, and peri-strut inflammation.

Spermatogenesis in the Microminipig

The microminipig has considerable potential as an animal model to evaluate general toxicity; however, there are few studies on the male reproductive system, particularly regarding spermatogenesis. The objectives of the present study were to clarify the stages of the seminiferous epithelium cycle on the basis of spermiogenesis and to determine the duration of spermatogenesis in the microminipig. Eleven microminipigs from 6 to 9 months of age were used for histological analyses. Spermiogenesis and stages of the seminiferous epithelium cycle were classified according to the degree of acrosomal development as shown by the periodic acid-Schiff reaction. Three of the animals were intravenously injected with 5-bromo-2-deoxyuridine to determine the duration of spermatogenesis by immunohistochemistry. Spermiogenesis was classified into 15 steps according to the morphological development of the acrosome, nucleus, and flagellum. The seminiferous epithelium cycle was classified into 11 stages based on the steps of spermatid development and germ cell associations. The length of the seminiferous epithelium cycle and the overall spermatogenesis process in the microminipig were estimated to be approximately 9.1 and 40.9 days, respectively. The results indicate the potential application of the microminipig in the evaluation of testicular toxicity, such as spermatogenesis disruption, in general toxicity studies.