Encapsulated Cell Technology

WENG TAO, MD, PHD

Encapsulated cell technology (ECT) was developed to treat diseases of the central nervous system (CNS)¹ and the eye.² ECT implants consist of living cells encapsulated within a semipermeable polymer membrane and supportive matrices. The encapsulated cells are genetically engineered to produce a specific therapeutic substance to target a specific disease or condition. Once surgically implanted into the CNS or eye, the semipermeable polymer membrane has 2 main functions: it allows the outward passage of the therapeutic product while protecting the encapsulated cells from rejection by the patient's immune system and also permits ready access to oxygen and nutrients.

MECHANISM OF ACTION

The inability to deliver biologically active molecules directly to their target sites is a major hurdle to their use in the treatment of CNS and eye diseases. The blood-retina barrier (BRB) prevents the penetration of most molecules to the neurosensory retina, in the same way the blood-brain barrier (BBB) hinders access to the CNS. ECT offers the potential for controlled, continuous, long-term delivery of therapeutics, including a wide variety of novel proteins and other compounds, directly to the retina, bypassing the BRB. In addition, the implants can be retrieved, providing an added level of safety. Thus, ECT has promising applications to major types of ocular disorders such as retinal degeneration, ocular inflammation, and angiogenesis.

An intraocular implantable encapsulated cell device prototype for chronic delivery of therapeutic agents has been developed to treat ophthalmic disorders (Figure). The device consists of genetically modified cells packaged in a hollow, semipermeable membrane. The hollow fiber membrane prevents immune molecules, eg, antibodies and host immune cells, from entering the device, while it allows nutrients and therapeutic molecules to diffuse freely across the membrane. The encapsulated cells continuously secrete therapeutic agents and derive nourishment from the host milieu. The device is implanted through a small pars plana incision and anchored to the sclera by a small titanium wire loop. The active intravitreal portion of the device measures approximately 1 mm in diameter and 6 mm in length. It is fixed outside the visual axis.

Advances in molecular biology over the last 2 decades have led to the discovery of potent proteins, including cytokines and neurotrophic factors. The potential therapeutic value of these molecules is impressive; however, despite promising results in short-term animal studies, few if any proteins have become successful therapeutics for human CNS or eye disorders because of barrier issues. A clinical trial of systemically administered ciliary neurotrophic factor (CNTF), a member of the interleukin (IL)-6 cytokine family with a molecular weight of 24 kilodaltons [kDa]) for amyotrophic lateral sclerosis (ALS) sponsored by Regeneron (Tarrytown, NY) is a good example. In this trial, despite high systemic doses, CNTF was undetectable in the CNS and there was no therapeutic benefit. In addition, the high peripheral CNTF levels were associated with major side effects, such as fever, fatigue, and blood chemistry changes that are consistent with activation of the acute phase response.^3,4 One reason for these results may be difficulty in achieving adequate concentrations of drug at the appropriate site; systemic administration may simply not be an effective way to treat CNS or ocular disorders. A continuous and site-specific delivery system may optimize the pharmacokinetics of these potential therapeutic agents in these 2 areas.

Encapsulated cell technology provides an alternative to the conventional means of administration. It is particularly attractive for the following reasons: (1) it allows potentially any therapeutic agent to be engineered into the cells and therefore has a broad range of applications; (2) the mammalian-cell�produced protein factor, freshly synthesized and released within the target site in situ, is more potent than the purified recombinant factors,⁵ and therefore, it reduces the dose requirement; (3) for proteins delivered directly into the cerebrospinal fluid (CSF) or eye, the limited CNS and eye volume of distribution, the presence of the BBB and BRB, and the low-dose requirement minimizes potential systemic toxicity associated with the protein; and (4) the cell-containing capsule can be retrieved.

Figure. Neurotech's proprietary encapsulated cell therapy (ECT). Encapsulated cell implants consist of living cells encapsulated within semi-permeable polymer membranes

ECT FOR OPHTHALMIC DISEASES

Neurotech USA (Lincoln, RI) is developing ECT for ophthalmic applications, primarily due to the many unmet medical needs in the field of ophthalmology. Although many topical pharmaceutical agents, such as antibiotics and anti-inflammatory agents, are available for the eye, few treatments, if any, are available for the common causes of vision loss that affect millions of people worldwide. Many of these devastating diseases are associated with the degenerating retina. Although previous studies have shown the promise of growth factors in reducing or halting the pathogenesis of retinal degeneration, unfortunately, progress has been slow in this field due to a number of challenges. First, the therapeutic agents that have shown promise cannot pass through the BRB. Second, repeated intraocular injection is not practical due to the chronic nature of these diseases. Third, an effective delivery system is not yet available.

Although there are a wide variety of eye diseases, 3 main clinical manifestations represent targets for therapy: photoreceptor degeneration in the neural retina, vascular proliferation, and inflammation. Several proteins show powerful neurotrophic, antiangiogenic, and anti-inflammatory properties. These proteins have the potential to significantly slow or halt retinal diseases. The lack of effective concentration at the target site, and the adverse effects associated with frequent intraocular injections are current challenges to administering these therapeutic proteins.

Delivery of Neurotrophic Factors for RP

NT-501 is an ECT-CNTF product that consists of encapsulated cells that secrete recombinant human CNTF. After implantation, CNTF is released from the cells constitutively into the vitreous gel. The NT-501 is manufactured to be sterile, nonpyrogenic, and retrievable. The current device is about 6 mm in length (including titanium loop) and will be placed well outside the visual axis in the human eye. This same device and device size has been used in preclinical toxicity and efficacy evaluation studies in dogs, pigs, and rabbits. The therapeutic intent of intraocular CNTF delivery is to reduce or arrest the progressive loss of photoreceptors, which is characteristic of retinitis pigmentosa (RP) and related retinopathies.

Retinitis pigmentosa is a group of incurable retinal degenerative diseases that have a complex molecular etiology. Approximately 100 000 Americans suffer from RP. More than 100 RP-inducing mutations have been identified in several genes including: rhodopsin, the rod visual pigment; peripherin, a membrane structure protein; and PDEB, the beta subunit of rod cyclic guanosine monophosphate (cGMP) phosphodiesterase. However, the genotype is unknown for the majority of patients. Regardless of the initial causative defects, however, the end result is photoreceptor degeneration. This common pathogenesis pathway provides a target for therapeutic intervention.

Many studies have demonstrated the promise of growth factors, neurotrophic factors, and cytokines as therapeutics for RP in short-term animal models. Among them, CNTF is reported to be the most effective in reducing retinal degeneration.¹⁵ Unfortunately, the local adverse effects associated with the intraocular administration of these factors at relatively high levels, their short half-lives following intravitreal administration, and the existence of the BRB, which precludes useful systemic administration of these agents for treatment of RP, have prevented their further clinical development and therapeutic practicality for RP patients. To circumvent these CNTF delivery problems, the NT-501 device has been developed. A phase 1 study of NT-501 demonstrated the safety and potential benefit of ECT-based delivery of CNTF.⁶

ECT-Neurotrophic Factors for Glaucoma

In addition to RP, ECT could be applied to neurodegeneration in glaucoma. This group of diseases is characterized by a progressive degeneration within the neural retina, eventually leading to blindness. Preserving photoreceptors and ganglion cells by slowing the degenerative process would have enormous therapeutic benefit, even if the underlying pathophysiology of disease were not corrected. The delivery of neurotrophic factors from encapsulated cells in the eye may significantly delay the loss of visual function associated with diseases in which degeneration plays a role. One of the most promising molecules that has shown significant efficacy in these models is CNTF. The efficacy of local administration of this agent has been demonstrated in several large animal models.

ECT-Antiangiogenic Factors for AMD and DR

Retinal vascular proliferation can occur in a number of different sites within the eye and plays a role in many ocular diseases. In age-related macular degeneration (AMD), angiogenesis of the choroidal vasculature can cause leakage of fluid and bleeding into the retina, subretina, retinal pigmented epithelium (RPE), and subneurosensory spaces, leading ultimately to loss of neural retinal elements. In diabetic retinopathy, neovascularization in the optic nerve and the retina leads to hemorrhaging within the vitreous cavity and subsequent retinal detachments from traction. In addition, angiogenesis within the iris (called rubeosis iridis) causes neovascular glaucoma. Delivery of antiangiogenic factors, either alone or in combination with neurotrophic factors, could have a significant impact on the progression of these diseases.

ECT-Anti-inflammatory Factors for Uveitis

Uveitis is a general term used to describe a group of syndromes that have, as a common feature, inflammation of the uveal tract. Experimental autoimmune uveoretinitis (EAU) can be induced in many animals by a subretinal injection of bovine serum albumin (BSA) or fibroblasts, although most studies have been done in rabbits. A number of nonsteroidal, anti-inflammatory proteins are known that may be valuable for long-term therapy of uveitis if delivered from encapsulated cells. Some anti-inflammatory factors that are believed to be promising for the treatment of uveitis diseases are inhibitors of inflammatory cytokines, such as antibodies or soluble receptors.

CONCLUSION

Validation of ECT technology in human eyes will be a critical step. If safety, efficacy, and consistent delivery of ECT are demonstrated in clinical trials, ECT could potentially serve as a delivery system for a number of ophthalmic diseases for which no effective therapies are currently available. RP

REFERENCES

1. Kordower JH, Winn SR, Liu YT,et al. The aged monkey basal forebrain: rescue and sprouting of axotomized basal forebrain neurons after grafts of encapsulated cells secreting human nerve growth factor. Proc Natl Acad Sci U S A. 1994;91:10898-10902.
2. Tao W, Wen R, Goddard MD, et al. Encapsulated cell-based delivery of CNTF reduces photoreceptor degeneration in animal models of retinitis pigmentosa. Invest Ophthalmol Vis Sci. 2002;43:3292-3298.
3. Cedarbaum JM; ALS CNTF Treatment Study (ACTS) Phase I-II Study Group. The pharmacokinetics of subcutaneously administered recombinant human ciliary neurotrophic factor (rHCNTF) in patients with amyotrophic lateral sclerosis: relation to parameters of the acute-phase response.Clin Neuropharmacol. 1995;18:500-514.
4. Cedarbaum JM; ALS CNTF Treatment Study (ACTS) Phase I-II Study Group. A phase I study of recombinant human ciliary neurotrophic factor (rHCNTF) in patients with amyotrophic lateral sclerosis. Clin Neuropharmacol. 1995;18: 515-532.
5. Hoane MR, Puri KD, Xu L, et al. Mammalian-cell-produced neurturin (NTN) is more potent than purified escherichia coli-produced NTN [In Process Citation]. Exp Neurol. 2000;162:189-193.
6. Sieving PA, Caruso RC, Tao W, et al. CNTF for human retinal neurodegeneration phase I trial of ciliary neurotrophic factor delivered by encapsulated cell intraocular implants for retinitis pigmentosa. Proc Natl Acad Sci. 2006;103:3896-3901.