Therapeutic Options for Retinoblastoma

Pia R. Mendoza, MD; Hans E. Grossniklaus, MD

Disclosures

Cancer Control. 2016;23(2):99-109. 

In This Article

Emerging Therapies

New developments in the treatment of retinoblastoma are geared toward an ideal system that is affordable and can easily be used by ophthalmologists or ocular oncologists — even those in developing nations — and is locally effective to the main tumor as well as tumor seeds, causes minimal complications, and can achieve a sustained treatment effect to retinoblastoma without spreading tumor cells. A sustained treatment effect is desired because, even when compounds do reach the target site, rapid clearance from the eye often results in short intraocular residence times. In addition, many patients with retinoblastoma are young children who oftentimes require sedation to receive intraocular therapy — sustained drug-release mechanisms will enable continuous medication dosing.

Preclinical models that accurately reflect human disease are needed for rare childhood cancers such as retinoblastoma because not enough patients exist for large-scale multicenter clinical trials.[8] Despite this lack of clinical trials, rates of patient survival and ocular preservation have improved.[7] Animal models remain instrumental in understanding biological mechanisms and designing targeted therapies for this disease.[8–10] Novel treatment options being explored in vitro and in vivo include alternative chemotherapeutic agents, molecularly targeted therapies, gene therapy, localized delivery, and the sustained-release of broad-spectrum chemotherapy agents.

Alternative Chemotherapeutic Agents

Topotecan, a topoisomerase inhibitor that works in the G0 phase of the cell cycle, may be used as a single agent or in combination with other agents for the treatment of retinoblastoma. Subconjunctival, sustained- release topotecan has been shown to decrease tumor burden in a transgenic mouse model of retinoblastoma.[69] In a similar fashion, periocular injections of topotecan in fibrin sealant in humans have been shown to achieve retinoblastoma tumor volume reduction.[70] Topotecan can also reduce retinoblastoma tumor burden when given systemically,[71] via IAC,[72] or via intravitreal injections.[43] When compared with standard chemotherapeutic agents used to treat retinoblastoma, topotecan avoids the retinal toxicity of melphalan, the nephrotoxicity of carboplatin, and the hematotoxicity of etoposide.[73] Topotecan is a good candidate drug for intravitreal injection because it has minimal retinal toxicity and achieves maximum concentration levels 50 times higher than systemic exposure.[73,74] Our laboratory has shown that 20-μg topotecan can provide sustained retinoblastoma cell-line killing in vitro,[75] similar to the clinically demonstrated retinoblastoma cell-line killing ability with 20-μg intravitreal topotecan.[43]

Anthracyclines are a class of chemotherapeutic drugs that inhibit topoisomerase and intercalate DNA. They are used to treat several cancers and are active against retinoblastoma; however, because of their limited blood–brain barrier penetration, their use is typically limited to extraocular and metastatic retinoblastoma. The anthracycline idarubicin has been shown to be effective against retinoblastoma tumor cells in bone marrow (response rate, 60%), but sufficient concentrations have not been achieved at target sites (eg, central nervous system).[76] A study to improve intraocular exposure of the anthracycline doxorubicin while also reducing systemic exposure to the drug was successful in maintaining sustained levels of doxorubicin after the intravitreal injection of the drug encapsulated in polyhydroxybutyrate microspheres in rabbits.[77]

Molecularly Targeted Therapy

RB1 was the first tumor suppressor gene identified and cloned, yet no effective, molecularly targeted cure currently exists for retinoblastoma.[78] This could perhaps be attributed to the variety of unknown secondary mutations and differential expression of other genes aside from RB1 involved in tumorigenesis, as well as questions on the cellular origin of retinoblastoma.[79,80] Significant progress has been made in our understanding of tumor biology, leading to the discovery and development of small molecules for the treatment of retinoblastoma, including novel, targeted therapeutics such as inhibitors of the MDMX–p53 response (nutlin-3a), histone deacetylase (HDAC) inhibitors, and spleen tyrosine kinase (SYK) inhibitors.[9,81]

Nutlin-3 is an imidazole analogue that plays a role in the activation of p53, a tumor suppressor protein. It is a small molecule inhibitor of MDM2 and MDMX and prevents the association of both these proteins with p53. Studies have shown that nutlin-3 restores the p53 pathway in retinoblastoma cells lacking both retinoblastoma protein and p53 activity, thus inducing p53-mediated apoptosis.[82] Subconjunctival injection of nutlin-3 in mouse models of retinoblastoma resulted in reduced tumor burden when used in combination with topotecan, demonstrating a synergistic effect.[83] Nutlin-3 is being studied in phase 1 clinical trials for the treatment of retinoblastoma.[82,83]

Although it is not expressed in normal human retinas, SYK was found to be upregulated in retinoblastoma tumor samples and has been shown to be required for retinoblastoma cell survival.[84] Inhibition of SYK with BAY-61-3606 and R406 caused cell death in retinoblastoma, and an in vivo study of orthotopic, xenograft mice showed that subconjunctival BAY-61-3606 was effective at blocking retinoblastoma cell proliferation.[84] In addition, studies of SYK inhibition have also implicated several downstream signaling molecules as mediators of SYK survival, including the B-cell chronic lymphocytic leukemia/lymphoma 2 (BCL2) family of proteins.[85] Myeloid cell leukemia 1, a member of the BCL2 family, is a potential target because it is upregulated in retinoblastoma and is being developed as a therapy for other cancers.[86]

Another class of targeted therapies in phase 1 clinical trials are the inhibitors of HDAC.[87] Cells with elevated E2F1 activity are uniquely sensitive to inhibitors of HDAC through the overexpression of proapoptotic factors. Cells lacking retinoblastoma protein have increased E2F1 activity, and retinoblastoma-derived cell lines have demonstrated particular sensitivity to apoptosis induced from inhibitors of HDAC.[88] In a preclinical trial, inhibitors of HDAC slowed the growth of retinoblastoma-derived tumors in both transgenic and xenograft murine models of retinoblastoma, with minimal off-target effects, suggesting that inhibitors of HDAC may specifically inhibit the proliferation of retinoblastoma tumor cells and, thus, have lower systemic toxicities relative to chemotherapeutic agents.[89]

Gene Therapy

Suicide gene therapy is based on the introduction of a viral or bacterial gene into tumor cells, thus allowing the conversion of a nontoxic compound into a lethal drug to kill the tumor cells. In a phase 1 study, intravitreal injections of an adenovirus vector containing HSVtk followed by treatment with ganciclovir were shown to be safe and effective against vitreous seeds.[90] Although these results were promising, it may be unlikely that this type of gene therapy could be used as first-line treatment for retinoblastoma but rather its use may be better suited as an adjunct to standard therapy for the treatment of refractory vitreous seeds.[90]

Local Drug-delivery Systems

Nanotechnology-based drug-delivery systems for the treatment of cancer have significantly evolved during the past decade by enabling options for site-specific delivery and increased bioavailability.[91] Investigations have been conducted on the potential use of different materials as intraocular drug carriers, such as biodegradable polyesters, dendrimers, liposomes, mesoporous silica, and gold nanoparticles.[91] These particles can be modified to target specific cells, and they can be designed to ensure sustained drug release to increase the therapeutic effectiveness of the drug molecules they contain.[92] Nanoparticle-based systems designed for the treatment of retinoblastoma have improved rates of drug delivery to the posterior segment of the eye and have increased the intravitreal half-life of chemotherapy agents, thus highlighting their potential in treatment of this cancer.[93] The intravitreal delivery of doxorubicin-loaded, polyester-based microspheres in the ocular tissue from rabbits exhibited reduced rates of toxicity to surrounding normal structures.[77] Dendrimer nanoparticles containing carboplatin significantly reduced tumor volume compared with free carboplatin in a retinoblastoma mouse model.[94] Polylactic glycolic acid nanoparticles developed to deliver doxorubicin[95] and etoposide[96] were tested on Y79 retinoblastoma cell lines and may be potential candidates for sustained drug-release models.

Gold-based nanoparticles have unique physical properties, including a strong ability to absorb near-infrared light, which enables the photothermal destruction of cancer cells.[97] Light-activated drug release can be attained using gold nanoparticles conjugated with chemotherapeutic agents.[98] For example, gold nanocages are encapsulated with a smart polymer, wherein the nanocages absorb light converted to heat, thus causing the collapse of the polymer and the subsequent release of doxorubicin.[99] The light-responsiveness of gold and other photosensitive nanocarriers infers considerable potential for ophthalmic conditions such as retinoblastoma because of the regular use of lasers in the treatment of retinal disease and the added benefit of facilitating the controlled timing and location of delivery of therapeutic agents. Gold nanoparticles can also cross the blood–retina barrier without significant rates of cytotoxicity.[100] The intravitreal injection of gold-tethered liposomes and viral-like nanoparticles containing topotecan is being investigated in a rabbit model of retinoblastoma that develops vitreous seeds.[55,101]

Another injectable agent being studied as a delivery system of chemotherapeutic agents for the treatment of retinoblastoma is the biodegradable carrier fibrin glue.[102] Carboplatin[103] and topotecan[69] released from fibrin depots have both been shown to retain their bioactivity against retinoblastoma cells in culture. Carboplatin released from a fibrin depot was also shown to decrease tumor burden in a transgenic murine model of retinoblastoma cells.[104] In another report, fibrin sealants sustained delivery of carboplatin in ocular tissues compared with carboplatin in plain solution, which clears rapidly in vivo.[105] Promising results have been reported for topotecan combined with fibrin in clinical studies.[71,106]

Suprachoroidal Injection

Suprachoroidal injection with microneedles is a route of targeted drug delivery to the choroid and retina that poses minimal risk of extraocular tumoral spread, because the vitreous and subretinal spaces are not entered. In a rabbit model, bevacizumab was determined to be safe and effective when injected into the suprachoroidal space; high concentrations were found in the posterior segment tissues.[107] Microneedles are 0.8- to 1.0-mm long, 32-gauge needles with a short bevel that penetrate through the sclera into the supraciliary space. When the microneedle injects compounds into the supraciliary space, these compounds posteriorly spread into the suprachoroidal space.[108]

In phase 1 clinical trials, microneedles for suprachoroidal injections have been used to inject triamcinolone into the suprachoroidal space in patients with uveitis and macular edema following retinal venous occlusion (NCT02303184, NCT01789320). The suprachoroidal delivery of chemotherapeutic agents is a promising approach for applications in retinoblastoma, and, ideally, will result in high choroidal and retinal concentrations targeted toward the elimination of subretinal seeds and the prevention of choroidal invasion, with minimal biodistribution and toxicity to surrounding tissue. Our laboratory is investigating the suprachoroidal and intravitreal injection of topotecan using microneedles in a rabbit model of retinoblastoma (Fig 2).[5]

Figure 2.

A–D. Intravitreal injection of topotecan in a rabbit model of retinoblastoma. (A) Vitreous seeds (asterisks) form in this retinoblastoma rabbit model. (B) A 32-gauge needle is inserted into the pars plana. (C) The needle (arrows) is inserted into its hub and topotecan is injected. (D) Vitreous seeds have disappeared after 3 weekly injections of 20 μg topotecan.
Reprinted from Grossniklaus HE. Retinoblastoma. Fifty years of progress. The LXXI Edward Jackson Memorial Lecture. Am J Ophthalmol. 2014;158(5):875–891, with permission from Elsevier.

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