Assessing Anticalcification Treatments in Bioprosthetic Tissue by Using the New Zealand Rabbit Intramuscular Model

The objective of this work was to demonstrate that the New Zealand White (NZW) rabbit intramuscular model can be used for detecting calcification in bioprosthetic tissue and to compare the calcification in the rabbit to that of native human valves. The rabbit model was compared with the commonly used Sprague–Dawley rat subcutaneous model. Eighteen rabbits and 18 rats were used to assess calcification in bioprosthetic tissue over time (7, 14, 30, and 90 d). The explanted rabbit and rat tissue discs were measured for calcium by using atomic absorption and Raman spectroscopy. Calcium deposits on the human valve explants were assessed by using Raman spectroscopy. The results showed that the NZW rabbit model is robust for detecting calcification in a shorter duration (14 d), with less infection complications, more space to implant tissue groups (thereby reducing animal use numbers), and a more metabolically and mechanically dynamic environment than the rat subcutaneous model . The human explanted valves and rabbit explanted tissue both showed Raman peaks at 960 cm⁻¹ which is representative of hydroxyapatite. Hydroxyapatite is the final calcium and phosphate species in the calcification of bioprosthetic heart valves and rabbit intramuscular implants. The NZW rabbit intramuscular model is an effective model for assessing calcification in bioprosthetic tissue.Abbreviations: BHV, bioprosthetic heart valve; FET, formaldehyde–ethanol–Tween 80; NZW, New Zealand WhiteBecause of their outstanding durability, mechanical heart valves were 1 of the first replacement valves used in humans. However, drawbacks to mechanical valves include patient complaints regarding the noise the valve makes and the risk of lifelong anticoagulation therapy.^4,20 Currently bioprosthetic heart valves (BHV) are 1 of the most common medical devices used to replace damaged mitral and aortic heart valves in men and women 65 y and older.¹⁷A leading drawback to BHV is dystrophic calcification, which is typically the primary failure mode for these devices.¹⁵ Calcification in BHV is believed to be caused by multiple factors, with glutaraldehyde (used to crosslink the tissue), residual cellular debris, and mechanical stress as the major factors. Patient factors including diabetes, renal failure, atherosclerosis, and calcium metabolism disorders can be important contributors as well.¹⁴ BHV are designed by using bovine pericardium, porcine valve leaflets, and porcine pericardium or dura mater; the most common tissues used are bovine pericardium and porcine valve leaflets.³⁶ Preventing or delaying calcification of BHV is an ongoing research dilemma of biomedical device companies. A 1982 study assessed explanted human valves that had been treated with glutaraldehyde only and found that calcification was the main cause of valve explantation and valves typically lasted only 9 1/2 y on average.³ To increase the durability and delay the onset of calcification of glutaraldehyde-treated valves, researchers and heart valve manufacturers began developing anticalcification treatments. In early studies, chemical compounds like protamine were used to block charged chemical groups and thus reduce calcification in glutaraldehyde-treated BHV.¹³ Other treatments used ethanol, AlCl₃, FeCl₃, L-Hydro, and osteopontin.^11,23,25,35 Many of these treatments were either proven ineffective or were not able to be manufactured. Effective new anticalcification treatments currently available commercially include α-amino oleic acid (Medtronic, Minneapolis, MN), an ethanol-based treatment (Linx, St Jude Medical, St Paul, MN), and a process involving formaldehyde, ethanol, and Tween 80 (ThermaFix, Edwards Lifesciences, Irvine, CA).^10,12 However, young adolescents and patients with calcium metabolic disorders still receive mechanical valves due to rapid calcification of BHV implants in these patient groups.²⁶An animal model is necessary to assess the effects of anticalcification treatments on bioprosthetic tissue in a short and effective timeframe. When assessing anticalcification tissue treatments, the animal models used are typically rats and rabbits. Wistar and Sprague–Dawley rats and New Zealand White (NZW) rabbits are the strains of choice.^5,29 Small animals are chosen because they are inexpensive, the tissue calcifies in a short period of time, the surgical procedure requires minimal pain and discomfort, and the animals are easy to care for.⁹ Typically, the BHV leaflet tissue is cut into discs or is used as whole leaflets and implanted subcutaneously in either rats or rabbits. Typical study duration to assess anticalcification efficacy in the rat model varies from 90 d to 6 mo.^24,41 The Sprague–Dawley rat is one of the most common models used to assess BHV tissue calcification properties.²⁷ The NZW rabbit model is another small animal model available, but to date only the subcutaneous route has been used for tissue biocompatibility and toxicology in drug efficacy studies only.Large animal models used to assess the safety and efficacy of BHV have included pig, sheep, cows, and nonhuman primates, but the primary model currently used is the juvenile sheep.³⁸ The juvenile sheep is a robust model that performs similarly to humans with regards to hemodynamics, valve annulus sizing, and thrombogenicity.^1,10 A typical safety and efficacy study in juvenile sheep lasts 20 wk.²¹ The drawbacks to using sheep to assess anticalcification treatments are the large sample size needed to demonstrate statistically significant differences, high surgical costs, and high animal-care costs.The age of the animal is important when assessing the calcification potential of BHV tissue. Compared with their older counterparts, juvenile animals demonstrate higher calcium metabolism because of bone-building, organ function, and structural tissues.⁷ Ways to assess calcium metabolism in small animals include measuring skeletal length and bone growth relative to sexual maturation.³² In Wistar rats, bone growth increased from 276 µm/d in 21-d-old weanlings to 330 µm/d in 35-d-old rats.^19,42 The bone growth spurt in the rats began to slow, falling to 85 µm/d by day 80, with full maturity by 24 wk. Compared with rats, rabbits mature more slowly, reaching maturity by 34 wk of age. A study involving 17 male and 12 female NZW rabbits assessed growth of the tibia and femur, assessing the correlation of tibial and femoral lengths and sexual maturity in rabbits.³⁹ This knowledge helps researchers assess the progressive growth and maturity of rabbits as they change from juvenile to adults.²⁸ This type of assessment may be important for understanding how intramuscular implants calcify in juvenile (6- to 8-mo-old) rabbits.The NZW rabbit intramuscular model offers a particular advantage for assessing tissue calcification properties. Epinephrine in rabbits had a higher absorbance and diffusion rate when injected intramuscularly compared with subcutaneously.¹⁶ The intramuscular region in rabbits has a rich vascular supply due to the high density in the latissimus dorsi compared with the subcutaneous of either rats or rabbits. The vascular transport mechanisms of muscle allow it to respond to foreign material (such as BHV tissue) more efficiently than the response to subcutaneous implants.⁴² The intramuscular region also offers a more mechanically dynamic environment than the static subcutaneous region, better mimicking some of the stresses on a tissue that a BHV tissue might endure when implanted in humans.Many analytical methods are used to assess the type of calcification that occurs in arteries or bioprosthetic tissue; currently gaining popularity is near-infrared Fourier transform Raman spectroscopy.³⁴ This method measures light scattered inelastically from photons.⁴⁰ Elastically scattered photons have the same energy (frequency) and therefore wavelength as the incident beam. However, a small fraction of light (approximately 1 in 10⁷ photons) is scattered at optical frequencies different from, and usually lower than, the frequency of the incident photons. This process of inelastic scattering of photons is called Raman scattering, which can occur with a change in vibrational, or rotational, or electronic energy of the molecule being studied. The difference in energy between the incident photon and the Raman-scattered photon is equal to the energy of a vibration of the scattering molecule, such as calcium or phosphate. A plot of intensity of scattered light versus energy difference is called a Raman spectrum. When used to assess the type of calcification in BHV, Raman spectroscopy has been shown to be effective in analyzing the presence of calcium phosphate species and, when combined with a calcium assay, the relative calcium:phosphate ratios.⁶ The most common type of calcification is the mineral hydroxyapatite Ca₁₀(PO₄)₆(OH)₂]. Roughly 70% of all bone is composed of hydroxyapatite.¹⁸ By using Raman spectroscopy (830 nm), hydroxyapatite was found at wavelengths of 960 to 1200 cm⁻¹ in human explanted BHV.³³ Other calcium–phosphate combinations found on explanted human heart valves include carbonate apatites, octacalcium, dicalcium, and amorphous calcium phosphates.⁸ Nascent calcification in biologic tissues goes through phase transformations of unstable calcium–phosphate salts to more stable calcium phosphate salts, ultimately maturing into hydroxyapatite.²² Raman spectroscopy can be applied to a variety of different morphologies, giving it a unique advantage when analyzing biologic samples, which are generally mixtures of fluids, tissues, and mineral deposits.³⁰ Raman analysis requires minimal preparation of biologic samples and is nondestructive to the sample. In the current study, near-infrared Fourier transform Raman spectroscopy was used to detect the presence of calcification in bioprosthetic valves explanted from humans and in bioprosthetic valve tissue from intramuscular rabbit explants.The objective of this study was to validate the rabbit intramuscular model for assessing the calcification potential of bioprosthetic tissue. The outcome of this study was to assess the length of study (days) necessary to see significant differences in calcification among 3 test groups and 1 control group, compare the type of calcification seen in the rabbit model with that of human valve explants (Raman spectroscopy), and to correlate the rabbit model to human BHV calcification.