Intracranial Administration of Adenovirus Expressing HSV-TK in Combination with Ganciclovir Produces a Dose-Dependent, Self-Limiting Inflammatory Response

Abstract

Replication-defective adenovirus expressing the herpes simplex thymidine kinase gene (H5.010RSVtk) may be useful in treating human gliomas. To determine the toxicity of this therapeutic strategy, we injected H5.010RSVtk stereotactically into the normal brain of Wistar rats, cotton rats, and rhesus monkeys in conjunction with systemic ganciclovir (GCV) at 10 mg/kg per day. In the Wistar rat, 5.7 × 10⁹ pfu resulted in histopathologic injury consisting of localized necrosis, mild gliosis, marked malacia, and focal astrocytosis; however, 1.0 × 10⁸ pfu resulted in only mild gliosis and trace meningitis and approximates a “no toxic effect” dose. A dose of 1.0 × 10⁹ pfu in both adenoviral immune and adenoviral naive cotton rats resulted in similar findings. In the rhesus monkey, doses ranging from 1.4 × 10⁸ pfu to 1.5 × 10¹¹ pfu resulted in localized gliosis, necrosis, perivascular cuffing, meningitis, and roughly correlated in severity with increasing dose. No histologic evidence of toxicity was found in non-central nervous system (CNS) tissues, and no virus could be cultured from cerebrospinal fluid (CSF), blood, urine, and stool samples. All animals survived to prescribed end points without signs of general toxicity or neurologic symptoms, except for 2 of the rhesus monkeys, one of which became febrile and the other of which developed a grand mal seizure (both subsequently resolved). These toxicology studies define the parameters for developing a phase I clinical trial. We examined the toxicity of replication-defective adenovirus expressing the herpes simplex thymidine kinase gene in conjunction with systemic ganciclovir (GCV) administration in the normal brain of Wistar rats, cotton rats, and rhesus monkeys. Magnetic resonance imaging (MRI), pathologic, and clinical signs of toxicity correlated roughly with increasing dose, with the highest doses producing significant histopathologic lesions with minimal clinical signs of toxicity. The lowest dose of vector corresponds to the lowest dose used in a phase I clinical trial, which is currently underway.