Dose dependent effects of cardiac β ₂ adrenoceptor gene therapy

Adenoviral-mediated gene transfer during cardiopulmonary bypass (CPB) achieves efficient myocardial transgene expression. The optimal vector dose required to produce not only increased β adrenoceptor (βAR) density but, more importantly, enhanced left ventricular (LV) function is unknown. In addition, it is unclear if absent extracardiac expression in preliminary studies represented cardiac specific, as opposed to selective gene delivery, as a consequence of low vector doses. Adenoviral vector encoding the human β ₂ adrenoceptor (Adeno-β ₂AR) was delivered to cardioplegic arrested hearts of neonatal piglets during CPB in three doses ranging from 5 × 10 ¹¹ total viral particles (tvp) to 2 × 10 ¹² tvp. Control animals received adenoviral vector encoding β galactosidase (Adeno-βgal) or PBS (PBS). LV and liver βAR density and in vivo LV function were assessed 5 days later. Elevated LV βAR density was present after delivery of Adeno-β ₂AR at all doses. Piglets which received 5 × 10 ¹¹ tvp and 1 × 10 ¹² tvp Adeno-β ₂AR demonstrated enhanced LV dP/dt _max but in those receiving 2 × 10 ¹² tvp LV dP/dt _max was unchanged. Moreover, at this higher dose of adenoviral vector the detrimental effects of cardiac inflammation and extracardiac gene overexpression became apparent. Although the highest increase in cardiac βAR density occurred after high-dose Adeno-β ₂AR, LV dP/dt _max was not enhanced. Moreover, significant extracardiac gene expression was present at this dose, emphasizing the need for careful dose response studies in gene therapy. However, cardiac selective β ₂AR overexpression does occur following adenoviral vector delivery during CPB and cardioplegic arrest resulting in enhanced LV dP/dt _max.