Our data demonstrate favorable in vivo distribution and efficacy of the telomerase inhibitor GRN163 when applied intranasally into human intracerebral glioblastoma xenografts. In our initial distribution study, we found rapid movement of GRN163 into the brain, including the brainstem and intracranial trigeminal nerves. This finding is consistent with the extraneuronal pathway that has been proposed for transport of therapeutic agents from the nasal cavity into the brain.2,3
This pathway occurs along the olfactory and trigeminal sensory neurons and likely involves extracellular bulk flow along perineuronal and/or perivascular channels, which delivers drug directly to the brain parenchyma, spinal cord, and perhaps the CSF ().3
Thorne et al. reported that insulin-like growth factor-1 is rapidly transported into the brain and upper spinal cord within 30 min of intranasal administration via the extra-neuronal pathway.3
Delivery along the extraneuronal pathway is not receptor-mediated and requires only minutes for the drug to reach the brain.5
It is unlikely that GRN163 is delivered via an intraneuronal pathway along the primary olfactory sensory neurons, because this pathway involves axonal transport and requires several hours to days for the drug to reach different areas of the brain.4,18
In our intracerebral tumor model, GRN163 showed tumor-specific distribution at all time points studied (0.5 up to 24 h) and achieved highest concentration within the tumor at 4 h after delivery, indicating favorable uptake and retention of the compound in the tumor. This may be due to the specific binding affinity of this compound to telomerase in the tumor cells,19
and the specificity achieved with intranasal delivery appears to be superior to the results obtained using CED, which reportedly delivers drugs beyond the tumor boundary into adjacent normal brain tissues.11,20,21
This spillover of drug to adjacent brain regions may be due to a pressure gradient of convective bulk flow of CED and could lead to neural toxicity.12,22
In contrast to CED, intranasal GRN163 showed very little or no accumulation in adjacent normal brain tissues surrounding the tumor. Thus, intranasal delivery may prevent unwanted damage to normal brain tissues adjacent to the tumor.
The results of our tumor efficacy study, in which rats were treated intranasally with a daily dose of 0.65 μmol of GRN163 for 12 days, showed that rats treated with GRN163 survived significantly longer than rats treated with PBS- and/or MM-Control. Notably, three rats treated with GRN163 showed no neurological symptoms by day 104 and no evidence of tumor at the original implantation site. Telomerase inhibition is related to telomere length in a time-dependent manner. Rapid inhibition occurs in cells with shorter telomeres,23,24
and U-251 MG cells have relatively short telomeres (approx. 3.9 kilobases) as compared with other brain tumor cells.11
Our in vitro studies showed that a 3-day exposure of U-251 MG cells to GRN163 inhibited telomerase activity by 81% in a dose-dependent manner. In addition, intranasal treatment of GRN163 for 12 days inhibited telomerase activity of intracerebral tumors by 54%. Thus, it is likely that in this study intranasal delivery of GRN163 led to specific inhibition of telomerase activity in U-251 MG tumor cells, which inhibited their growth, resulting in prolonged lifespan for the animals. The length of the protocol used in this study was determined by the average lifespan of the control animals, which limited the time frame within which results from treatment could be considered significant to approximately 12 days. We used the maximum dose of GRN163 (0.65 μmol/65 μl) that could be dissolved in saline solution. Further work will be required to fully define the maximum tolerated dose and dosing schedule for optimal intranasal delivery. These parameters will likely be determined by the size of the tumor, the characteristics of the drug, and the type of animal model studied.
Recently, intranasal delivery of insulin has been shown to improve memory in normal adults25
and in patients with early Alzheimer’s disease26
without changing blood levels of glucose or insulin,27
implying that insulin entered the brain directly without entering the bloodstream. Intranasal treatment with perillyl alcohol given to patients with recurrent brain tumors is being investigated in combination with radiotherapy.28
In conclusion, this is the first report to demonstrate that intranasal delivery of telomerase inhibitor GRN163 achieves efficient distribution into an intracerebral tumor and inhibits tumor growth in vivo, resulting in prolonged survival of athymic rats without apparent toxicity. Our findings support further development of intranasal GRN163 as a potential therapy for brain-tumor patients and perhaps as a means for treating multifocal brain tumors and/or pediatric brainstem tumors, which are less amenable to potentially risky surgical procedures. Our results also support the investigation of delivering other tumor-specific agents intranasally for the treatment of intracranial neoplasms.