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(i9) United States
(12) Patent Application Publication oo) Pub. No.: US 2006/0234369 Al
Sih (43) Pub. Date: Oct. 19,2006
Patent Application Publication Oct. 19,2006 Sheet 1 of 2
US 2006/0234369 Al
Patent Application Publication Oct. 19, 2006 Sheet 2 of 2 US 2006/0234369 Al
CROSS-REFERENCE TO RELATED
 This application claims the benefit of U.S. Provisional Application No. 60/671,140, filed on Apr. 14, 2005, under 35 U.S.C. § 119(e), which is hereby incorporated by reference.
 This application relates generally to implantable biosensors and, more particularly, to devices and methods which employ genetically modified electrically active cells to detect physiological events.
 Determining serum levels of analytes such as signaling molecules (e.g., hormones) normally entails withdrawing a blood sample from the patient, and then analyzing the sample on the benchtop. Obviously, this approach has limitations regarding the frequency of measurement and the inconvenience and discomfort associated with periodic blood draws. Optimization of implantable biosensors in order to allow continuous analyte measurement would lead to better monitoring of several human disorders such as diabetes mellitus. For example, if patients with diabetes were able to continuously see a display of glucose concentration in blood or tissue, they could better avoid extremes of glycemia and reduce their risk for long term complications. However, fibrosis of the foreign body capsule that typically develops around the implanted sensors 3-4 weeks after implantation may reduce the influx of substrates such as glucose and oxygen (Ward and Troupe, ASAIO J., 45:555 (1999); Updike et al. Diabetes Care, 23:208 (2000); Gilligan et al. Diabetes Care, 17:884 (1994)).
 One of the fundamental tasks required of implantable medical devices is accurate real-time determination of relevant functional physiological needs. For instance, a cardiac pacemaker must determine the pacing rate required to supply the body with adequate cardiac output. Biosensors that transduce biological actions or reactions into signals amenable to ready detection and/or processing are well suited for such monitoring (Pancrazio et al, Ann. Biomed. Eng., 27:697 (1999)). Nonetheless, typical in vivo biosensors only approximate physiological function via the measurement of surrogate signals and so may introduce a prime source of error in biological monitoring (Celiker et al. Pacing Clin. Electrophysiol, 21:2100 (1998); Moura et al. Pacing Clin. Electrophysiol., 10:89 (1987)).
 An alternative approach is to use a biologically based system that can sense physiological signals directly, thereby avoiding the approximation errors associated with surrogate signal sensing. Recently, the development of such a tissue-based biosensor was reported in which the endogenous signaling pathways of excitable tissue was exploited to couple the detection of in vivo circulating physiological inputs to a functionally responsive electrical output (Christini et al. Am. J. Physiol. Heart Circ. Physiol, 280:H2004 (1999)). Specifically, the activity and regulation of remotely engrafted neonatal cardiac tissue in a murine model system was monitored. The chronotropic dynamics of the exogenous excitable cardiac allografts were highly correlated
with the activity of the endogenous heart. Moreover, pharmacological studies in this model system showed that the transplanted allografts were regulated by circulating catecholamines.
 What is needed is a biologically based biosensor to detect particular molecules, e.g., circulating molecules.
 The present invention provides implantable biosensors. The biosensors of the invention include donor tissue or cells, optionally transgenic (genetically altered) donor tissue or cells, that are electrically excitable or are capable of differentiating into electrically excitable tissue or cells, such as cardiac, neural or skeletal muscle donor tissue or cells, and are capable of binding a particular physiological molecule, which binding in turn produces a biological signal that can be detected (monitored) and, in one embodiment, correlated with the presence and/or amount of one or more physiological molecules. In one embodiment, the donor tissue or cells are xenogeneic relative to the intended recipient mammal, e.g., human, mouse, rat, pig, rabbit, sheep, bovine, horse, dog or cat. Such a biosensor may be used to repeatedly and chronically track changes in soluble molecules found in physiological fluid, e.g., in the blood, of an animal, e.g., a mammal. The invention thus provides for automatic measure of circulating molecules by an implantable system, in contrast to the infrequent and inconvenient monitoring of various circulating molecules by benchtop analysis of blood samples. In one embodiment, the invention provides for automatic monitoring of circulating molecules with a system including donor tissue or cells electrically connected to an implantable device having pacing/sensing capabilities, which can provide information that may optimize cardiac pacing or defibrillation therapy in contrast to current implantable pacemakers and defibrillators that do not have the capacity to measure circulating molecule concentrations. Moreover, the information obtained from the device may be automatically recorded and changes in the concentration of circulating molecules may be monitored so as to allow physicians to better manage patient health.
 In one embodiment, the biosensor includes transgenic donor tissue or cells. The transgenic donor tissue or cells include an expression cassette comprising a transcriptional regulatory element operably linked to an open reading frame encoding a gene product, e.g., a protein, which is capable of being associated with the cell membrane and binding a molecule (ligand) found in physiological fluid. In one embodiment, the transgenic donor tissue or cells are electrically excitable tissue or cells and the binding of one or more ligands to the gene product alters the amount and/or activity of one or more intracellular second messengers. The alteration in the amount and/or activity of one or more intracellular second messengers in turn modulates the electrical potential of the transgenic donor tissue or cells. For example, binding of ANP or BNP to their receptor alters the amount of the intracellular second messenger cGMP; binding of glucagon to its receptor alters the amount of the intracellular second messenger cAMP; binding of catecholamines, e.g., epinephrine, norepinephrine, or other adrenergic receptor ligands to an adrenergic receptor alters the amount of the intracellular second messenger inositol triphosphate; binding of a ligand to a muscarinic receptor alters the amount of, for instance, inositol triphosphate and/or dia