Timeline for developing TLD1433 4.3.2. Pre-clinical studies and device development.
While this review focuses primarily on the design and development of TLD1433 from chemical and photophysical principles, it is important to point out the enormous effort that went into its pre-clinical development in partnership with TLT, plus the engineering of the proprietary medical laser system (TLC-3200) and the dosimetry fiber optic cage (TLC-34000) that are being used in the clinical trial. Many animal models and testing of increasing levels of sophistication were required to establish confidence in the technology and satisfy various approval requirements. The development of the complete PDT package based on TLD1433 as the photosensitizer was a multidisciplinary effort that required the expertise and contributions from chemists, biologists, medical biophysicists, engineers, clinicians, investors, industrial partners, and lawyers.
4.3.3. Clinical trial.
In March 2017, TLD1433 was administered to the first patient in a human clinical trial (ClinicalTrials.gov Identifier: NCT03053635), “Intravesical Photodynamic Therapy (PDT) in BCG Refractory High-Risk Non-muscle Invasive Bladder Cancer (NMIBC) Patients”, sponsored by TLT. The trial was carried out at the University Health Network (Toronto, Ontario, Canada) under principal investigator Dr. Girish Kulkarni with Dr. Michael Jewett as Chairman of TLT’s Medical and Scientific Advisory Board. It was a phase lb, open-label, single-arm, single-center study on patients with NMIBC (Ta, Ti, and/or Tis) who had either refused or were not candidates for a radical cystectomy.
The study plan consisted of nine participants, each assigned to one of two phases. (1) Three subjects receive PDT at half of the projected therapeutic dose of TLD1433 (0.35 mg cm−2) and are monitored for safety and tolerability. (2) If the treatment of the first three patients in the first phase does not raise safety concerns after one month of patient follow-up (based on the judgement of a safety monitoring committee), then six subjects receive PDT with the full therapeutic dose of TLD1433 (0.70 mg cm−2) and are monitored for 180 days.
The primary endpoint of the trial was an evaluation of safety and tolerability, assessed by the incidence and severity of adverse effects (up to completion of the follow-up phase at 180 days). The secondary endpoint was the determination of pharmacokinetics as the maximum observed concentration (Cmax) of TLD1433 in the blood and urine, as well as the area under the curve from time zero to the last quantifiable concentration (AUCO-t). The exploratory endpoint was efficacy, which was assessed in terms of recurrence and survival. The recurrence endpoint was either recurrence-free survival rate at three and six months or recurrence rate at three and six months. The survival endpoint was either overall survival during the study, or overall survival rate at three and six months.
The protocol used in this study is available at ClinicalTrials.gov. Under general anesthesia, patients were infused with TLD1433 (directly into the bladder) for one hour. The light dose was then delivered after TLD1433 had been completely rinsed from the bladder. Removal of unbound TLD1433 followed by uniform illumination of the entire bladder was possible because TLD1433 is almost 2OO× more selective for bladder tumors than normal, healthy urothelial tissue. TLD1433 was supplied as a lyophilizate, packaged in amber borosilicate glass vials stored at room temperature, and was reconstituted in sterile water just before administration to obtain the final clinical dilution determined by bladder volume. Patients were required to restrict fluid intake for twelve hours before TLD1433 administration. Before instillation, a transurethral catheter was inserted and the bladder was drained. TLD1433 was then infused intravesically for one hour, followed by three washes with sterile water. The bladder was then distended with a fourth instillation of sterile water to prevent any folds that would compromise uniform light delivery. Next, an optical fiber with a spherical diffuser was positioned in the center of the bladder, and irradiance sensors were placed via a cage on the bladder wall surface. The assembly was locked in place with an endoscope holder. The irradiance (mW cm−2) was integrated at all sensors, until the target radiant exposure of 90±9 J cm−2 was achieved, and then the laser was turned off. The total irradiation time depended on the bladder size and the tissue optical properties of the individual bladders.
The first of the three patients in the first phase of the study was treated March 30, 2017. The primary, secondary, and exploratory (at 90 days post-treatment) endpoints were successfully achieved for all three patients treated with the maximum recommended starting dose. At 180 days post-treatment, the three patients treated with the sub-therapeutic dose of TLD1433 recurred, although there was no sign of progression.
The fourth patient, and the first of the six patients to receive the therapeutic dose of TLD1433 in the second phase of the study, was treated on August 1, 2017. The primary, secondary, and exploratory (at 90 days post-treatment) endpoints were achieved, but this patient presented with metastatic urothelial carcinoma 138 days post-treatment (presumably due to disseminated bone micrometastases present at the time of treatment).
The clinical procedure was optimized (details not disclosed) commencing with the fifth patient. Patients five and six were treated in January and February of 2018, respectively, and met the established primary, secondary, and exploratory 90-day endpoints with no evidence of tumor recurrence. While an additional three patients were part of the original trial design, TLT’s Medical and Scientific Advisory Board unanimously voted for early termination of the study in May 2018 based on successfully achieving the primary and secondary endpoints (and exploratory endpoint at 90 days) in six patients. Since that time, patients five and six also met the exploratory efficacy endpoint, with no evidence of disease at 180 days. The next step is an international, multi-center phase II study for NMIBC with efficacy as the primary endpoint in a much larger patient population.
4.4. Future direction
The future is bright for transition metal complexes and PDT/PCT, and many research groups are demonstrating the potential of Ru(II) compounds in this application. Areas to watch include (1) the creation of photosensitizers and photo-sensitizer-vehicle conjugates that are highly selective for tumors over normal tissue (yet general enough to be used on multiple cancer types), offering improved safety margins for systemic delivery, (2) the design of x-ray activatable photosensitizers that exploit the best attributes of both radiotherapy and PDT for hard-to-treat tumors, and (3) the development of PDT/PCT regimens that stimulate antitumor immunity, which would move PDT/PCT from being viewed as a local treatment to one that can prevent or even target metastatic tumors. A few illustrations involving TLD1433 are highlighted below.
4.4.1. Rutherrin®.
Despite light-mediated cancer therapy being inherently selective by confining the light treatment to malignant tissue, intravenous (IV) delivery of previous photosensitizers has caused unwanted side effects due to off-target accumulation. Thus, there is a continued need to develop better selectivity strategies for photosensitizers that will be administered systemically. There has been ongoing interest by a number of research groups in the use of the protein transferrin (Tf) to carry metal-based drugs as cargo to Tf-receptors that tend to be overexpressed on cancer cell surfaces.152–158 Ru(II) transition metal complexes, including photosensitizers, have been shown to exhibit nonselective binding to both holo- and apo-Tf. This is also the case for TLD1433 and its derivatives, where Tf-binding both enhances and red-shifts the molar extinction coefficients of some of these photosensitizers under certain conditions. TLT has demonstrated that the photophysical properties of TLD1433 are improved by premixing TLD1433 and Tf, which includes reduced photobleaching, and that overall PDT efficacies are improved with a significant decrease in toxicity.154 The TLD1433-Tf conjugate was named Rutherrin®, with a Canadian patent pending, and is currently under clinical development for glioblastoma multiforme (GBM) and non-small cell lung cancer (NSCLC). Rutherrin® is able to cross the blood brain barrier (BBB) when sys-temically delivered to rats, with higher uptake by GBM cells relative to normal brain tissue. Activation of Rutherrin® with 808 nm light improved survival in this very aggressive animal model of GBM. They have also demonstrated that Rutherrin® can be activated by x-rays (20 Gy, 225 keV), and the next step is to investigate whether GBM tumors can be safely and effectively destroyed when Rutherrin® is activated transcranially with x-rays. Such developments have the potential to change the way brain tumors are treated, and to improve overall survival for what are now terminal diagnoses.
4.4.2. Immunomodulating PDT/PCT.
Antitumor immune responses, if successfully established, can protect against existing as well as relapsing cancer cells. Recently, certain photosensitizers and PDT regimens have been recognized for their capacity to train a host’s immune system against cancer and promote the development of antitumor immunity.34–38,159,160 Such PDT-induced antitumor immune responses have the capacity to target cancer cells at local sites and metastatic niches, and thus hold the key to establishing long-term cancer-free health. As such, the development of novel photosensitizers and regimens of immunomodulatory potential represent the frontier in the field of PDT (and PCT) research, and promise to yield the next generation of cancer immunotherapeutics. TLD1433 and its PDT regimen have been shown to induce antitumor immunity in a mouse model of colon cancer, and there is hope that this could translate to humans. We are actively developing other immunomodulating transition metal complexes and PDT/PCT regimens for melanoma specifically. As chemotherapy and radiotherapy are ineffective toward melanoma, outcomes could be improved with better adjuvant therapies that can be administered alongside surgery.
5. CONCLUSIONS
PDT activity has been known generally for over a century and as a cancer treatment for nearly half as long, and recent developments have demonstrated remarkable potency. However, no light-activatable prodrugs have emerged as a mainstream cancer treatment. This review and others have discussed the major obstacles to introducing PDT as a viable alternative to conventional cancer therapy approaches (e.g., surgery, radiotherapy, and chemotherapy). These major obstacles are: (1) its absolute dependence on molecular oxygen, (2) the paucity of photosensitizers that can be activated by tissue-penetrating near-infrared light, (3) poor or zero tumor selectivity for systemically-delivered photosensitizers (especially first-generation photosensitizers), (4) the inability to treat metastases using the current protocols, which are optimized for primary tumor ablation, (5) the lack of randomized, controlled clinical trials of adequate power, (6) the equipment required, although relatively inexpensive, is not standard clinical infrastructure, (7) the use of different treatment protocols that prohibit the comparison of treatment outcomes in small studies across different centers, (8) the lack of commitment and funds for PDT research, (9) the fact that the first-generation photosensitizer Photofrin® is still used in almost one-third of recent trials, and (10) the pervasive photosensitizer-centered approach to the design of next-generation photosensitizers for PDT rather than the development of complete and optimized PDT packages.
While there is no “ideal” photosensitizer, those derived from transition metal complexes offer many advantages. First, metal-based compounds can adopt a larger number of oxidation states compared to their organic counterparts, which allows a variety of bonding modes and geometries. The structural and chemical space that can be sampled with minor modification is vast. Second, inorganic complexes possess a much wider range of accessible excited-state electronic configurations, with characteristic photophysical and photochemical properties. They readily participate in energy- and electron-transfer processes upon photoexcitation, yet can remain very kinetically stable. Finally, coordination complexes have a modular architecture, whereby photophysical and chemical properties can be tuned through judicious choice of metals and ligands to achieve potent photobiological effects. As such, they have been of particular interest as systems that can yield PDT effects at low oxygen tension, operate via oxygen-independent photochemical processes for PCT, and/or be activated with tissue-penetrating near-infrared light. When designed from a tumor-centered approach, they can also stimulate important immunological responses.
The potential of transition metal complexes for PDT/PCT has been demonstrated in a number of Ru(II) polypyridyl systems investigated as in vitro photobiological agents. One example is TLD1433, which is the first Ru(II)-based photosensitizer for PDT to enter a human clinical trial. This system exploits long-lived triplet 3IL and 3ILCT states for 1O2 sensitization and for electron-transfer pathways, respectively, producing extremely potent photocytotoxic effects. It also exploits 3MLCT states that luminesce brightly in cancer cells and tumors, giving this photosensitizer an added theranostic capacity. Its design emerged from a knowledge of fundamental photophysical and chemical principles that were derived from the SARs of a large number of transition metal complexes studied in a standardized phenotypic in vitro (photo)cytotoxicity assay.
The standardization of this assay was key to comparing different photosensitizers. It is well known that there is a problem with reproducibility of biological results between different laboratories. With PDT/PCT, this problem is exacerbated by the added variables associated with light delivery and dosimetry. The solution is to screen compounds of interest and reference photosensitizers through a standardized assay in-house, rather than relying on published data for comparison. This approach has enabled us to make quantitative comparisons of the performance of our photosensitizers against others, which ultimately made the case for investing the time and money to move TLD1433 forward.
TLD1433 progressed from the bench to a clinical trial in six years thanks to efforts of a highly productive and motivated multidisciplinary team of chemists, biologists, medical biophysicists, engineers, clinicians, investors, industrial partners, and lawyers. The early identification of the target indication — NMIBC — facilitated the parallel development of the compound, the medical device, and the PDT package via a tumor-centered approach. The creation of a photosensitizer is only one moving part in a much bigger machine, and researchers making these compounds ought not lose sight of the big picture.
The phase Ib study of TLD1433, focused on safety and tolerability, was deemed a success, and a much larger, multicenter phase II study with efficacy as the primary endpoint is being planned. We hope that this development process, i.e., as part of a complete PDT package, might serve as a model for bringing improved transition metal complex photosensitizers to clinical studies. The key is to capitalize on the strengths of a multidisciplinary team, and to identify the right photosensitizer and the right light protocol for a target clinical indication.