A pharmacokinetic model for radioimmunotherapy delivered through cerebrospinal fluid for the treatment of leptomeningeal metastases

Astatine-211 based radionuclide therapy: Current clinical trial landscape

Astatine-211 (²¹¹At) has physical properties that make it one of the top candidates for use as a radiation source for alpha particle-based radionuclide therapy, also referred to as targeted alpha therapy (TAT). Here, we summarize the main results of the completed clinical trials, further describe ongoing trials, and discuss future prospects.

Phase 1 study of intraventricular 131I-omburtamab targeting B7H3 (CD276)-expressing CNS malignancies

BACKGROUND

The prognosis for metastatic and recurrent tumors of the central nervous system (CNS) remains dismal, and the need for newer therapeutic targets and modalities is critical. The cell surface glycoprotein B7H3 is expressed on a range of solid tumors with a restricted expression on normal tissues. We hypothesized that compartmental radioimmunotherapy (cRIT) with the anti-B7H3 murine monoclonal antibody omburtamab injected intraventricularly could safely target CNS malignancies.

PATIENTS AND METHODS

We conducted a phase I trial of intraventricular ¹³¹I-omburtamab using a standard 3 + 3 design. Eligibility criteria included adequate cerebrospinal fluid (CSF) flow, no major organ toxicity, and for patients > dose level 6, availability of autologous stem cells. Patients initially received 74 MBq radioiodinated omburtamab to evaluate dosimetry and biodistribution followed by therapeutic ¹³¹I-omburtamab dose-escalated from 370 to 2960 MBq. Patients were monitored clinically and biochemically for toxicity graded using CTCAEv 3.0. Dosimetry was evaluated using serial CSF and blood sampling, and serial PET or gamma-camera scans. Patients could receive a second cycle in the absence of grade 3/4 non-hematologic toxicity or progressive disease.

RESULTS

Thirty-eight patients received 100 radioiodinated omburtamab injections. Diagnoses included metastatic neuroblastoma (n = 16) and other B7H3-expressing solid tumors (n = 22). Thirty-five patients received at least 1 cycle of treatment with both dosimetry and therapy doses. Acute toxicities included < grade 4 self-limited headache, vomiting or fever, and biochemical abnormalities. Grade 3/4 thrombocytopenia was the most common hematologic toxicity. Recommended phase 2 dose was 1850 MBq/injection. The median radiation dose to the CSF and blood by sampling was 1.01 and 0.04 mGy/MBq, respectively, showing a consistently high therapeutic advantage for CSF. Major organ exposure was well below maximum tolerated levels. In patients developing antidrug antibodies, blood clearance, and therefore therapeutic index, was significantly increased. In patients receiving cRIT for neuroblastoma, survival was markedly increased (median PFS 7.5 years) compared to historical data.

CONCLUSIONS

cRIT with ¹³¹I-omburtamab is safe, has favorable dosimetry and may have a therapeutic benefit as adjuvant therapy for B7-H3-expressing leptomeningeal metastases.

TRIAL REGISTRATION

clinicaltrials.gov NCT00089245, August 5, 2004.

Modulation of solute diffusivity in brain tissue as a novel mechanism of transcranial direct current stimulation (tDCS)

The breadth of brain disorders and functions reported responsive to transcranial direct current stimulation (tDCS) suggests a generalizable mechanism of action. Prior efforts characterized its cellular targets including neuron, glia and endothelial cells. We propose tDCS also modulates the substance transport in brain tissue. High resolution multiphoton microscopy imaged the spread across rat brain tissue of fluorescently-labeled solutes injected through the carotid artery after tDCS. The effective solute diffusion coefficient of brain tissue (D_eff) was determined from the spatio-temporal solute concentration profiles using an unsteady diffusion transport model. 5–10 min post 20 min–1 mA tDCS, D_eff increased by ~ 10% for a small solute, sodium fluorescein, and ~ 120% for larger solutes, BSA and Dex-70k. All increases in D_eff returned to the control level 25–30 min post tDCS. A mathematical model for D_eff in the extracelluar space (ECS) further predicts that this dose of tDCS increases D_eff by transiently enhancing the brain ECS gap spacing by ~ 1.5-fold and accordingly reducing the extracellular matrix density. The cascades leading ECS modulation and its impact on excitability, synaptic function, plasticity, and brain clearance require further study. Modulation of solute diffusivity and ECS could explain diverse outcomes of tDCS and suggest novel therapeutic strategies.

IntraOmmaya compartmental radioimmunotherapy using 131I-omburtamab-pharmacokinetic modeling to optimize therapeutic index

PURPOSE

Radioimmunotherapy (RIT) delivered through the cerebrospinal fluid (CSF) has been shown to be a safe and promising treatment for leptomeningeal metastases. Pharmacokinetic models for intraOmmaya antiGD2 monoclonal antibody ¹³¹I-3F8 have been proposed to improve therapeutic effect while minimizing radiation toxicity. In this study, we now apply pharmacokinetic modeling to intraOmmaya ¹³¹I-omburtamab (8H9), an antiB7-H3 antibody which has shown promise in RIT of leptomeningeal metastases.

METHODS

Serial CSF samples were collected and radioassayed from 61 patients undergoing a total of 177 intraOmmaya administrations of ¹³¹I-omburtamab for leptomeningeal malignancy. A two-compartment pharmacokinetic model with 12 differential equations was constructed and fitted to the radioactivity measurements of CSF samples collected from patients. The model was used to improve anti-tumor dose while reducing off-target toxicity. Mathematical endpoints were (a) the area under the concentration curve (AUC) of the tumor-bound antibody, AUC [C_IAR(t)], (b) the AUC of the unbound "harmful" antibody, AUC [C_IA(t)], and (c) the therapeutic index, AUC [C_IAR(t)] ÷ AUC [C_IA(t)].

RESULTS

The model fit CSF radioactivity data well (mean R = 96.4%). The median immunoreactivity of ¹³¹I-omburtamab matched literature values at 69.1%. Off-target toxicity (AUC [C_IA(t)]) was predicted to increase more quickly than AUC [C_IAR(t)] as a function of ¹³¹I-omburtamab dose, but the balance of therapeutic index and AUC [C_IAR(t)] remained favorable over a broad range of administered doses (0.48-1.40 mg or 881-2592 MBq). While antitumor dose and therapeutic index increased with antigen density, the optimal administered dose did not. Dose fractionization into two separate injections increased therapeutic index by 38%, and splitting into 5 injections by 82%. Increasing antibody immunoreactivity to 100% only increased therapeutic index by 17.5%.

CONCLUSION

The 2-compartmental pharmacokinetic model when applied to intraOmmaya ¹³¹I-omburtamab yielded both intuitive and nonintuitive therapeutic predictions. The potential advantage of further dose fractionization warrants clinical validation.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov , NCT00089245.

GD2-targeted immunotherapy and radioimmunotherapy

Ganglioside GD2 is a tumor-associated surface antigen found in a broad spectrum of human cancers and stem cells. They include pediatric embryonal tumors (neuroblastoma, retinoblastoma, brain tumors, osteosarcoma, Ewing sarcoma, rhabdomyosarcoma), as well as adult cancers (small cell lung cancer, melanoma, soft tissue sarcomas). Because of its restricted normal tissue distribution, GD2 has been proven safe for antibody targeting. Anti-GD2 antibody is now incorporated into the standard of care for the treatment of high-risk metastatic neuroblastoma. Building on this experience, novel combinations of antibodies, cytokines, cells, and genetically engineered products all directed at GD2 are rapidly moving into the clinic. In this review, past and present immunotherapy trials directed at GD2 will be summarized, highlighting the lessons learned and the future directions.

Pharmacokinetic delivery and metabolizing rate of nicardipine incorporated in hydrophilic and hydrophobic cyclodextrins using two-compartment mathematical model

The dispersion routes of cyclodextrin complexes with nicardipine (NC), such as hydrophilic hydroxypropyl-β-cyclodextrin (NC/HPβCD) and hydrophobic triacetyl-β-cyclodextrin (NC/TAβCD), through the body for controlled drug delivery and sustained release have been examined. The two-compartment pharmacokinetic model described the mechanisms of how the human body handles with ingestion of NC-cyclodextrin complexes in gastrointestinal tract (GI), distribution in plasma, and their metabolism in the liver. The model showed that drug bioavailability was significantly improved after oral administration of cyclodextrin complexes. The mathematical significance of this study to predict nicardipine delivery using pharmacokinetic two-compartment mathematical model with linear ordinary differential equations (ODE) approach represents a valuable tool to emphasize its effectiveness and metabolizing rate and diminish the side effects.

Bottom up Nano-particle Formation via Controlled Crystallization and Chemical Reactions

Many manufacturing techniques to produce nano-materials via a “bottom-up” approach are currently being developed and evaluated. The PureNanoTM platform technology developed by Microfluidics International Corporation (MFIC) has proven to be both an effective and energy efficient method to produce nano-scale entities including emulsions in addition to suspensions. This nano-manufacturing platform utilizes crystallization, precipitation and chemical reaction methods that produce nano-particles with specified size distributions and a desired morphology. The solids formed can be either amorphous or crystalline, which may exist in numerous polymorphs. In many cases the ability to obtain a specific composition (single species or mixture) is possible via careful selection and implementation of key processing conditions. The methods are based on controlling the local degree of super-saturation (SS) and/or stoichiometry during their formation and subsequent configuration and growth, when appropriate. To accomplish this, operational strategies and innovative processing techniques are coupled with qualitative insight into the basic mechanisms involved with these processes. Validation of the technology at the bench scale for crystallization, emulsions/cargo loading, and multi-phase reactions (interfacial and homogeneous) provided the justification to develop commercial scale systems. Examples are given here for crystallization of drugs for the pharmaceutical industry, a catalyst formed by deposition of metallic crystals on a carbon substrate, production of fine chemicals via emulsion formation for multi-phase reactions, and a homogeneous substitution reaction forming an insoluble product. Nano-materials with median particle size as low as 50 nm were produced. With respect to the drug particles, they were highly crystalline, of a single polymorph and pure. In all cases, results indicate both process performance enhancement and product quality/functionality improvements compared to materials produced with conventional methods, with at least 1-2 order of magnitude increases in surface/interfacial area and reduced energy needs. Furthermore, the technology is suitable for current Good Manufacturing Practices (cGMP) manufacturing.

Enhanced Transport Capabilities via Nanotechnologies: Impacting Bioefficacy, Controlled Release Strategies, and Novel Chaperones

Emerging nanotechnologies have, and will continue to have, a major impact on the pharmaceutical industry. Their influence on a drug's life cycle, inception to delivery, is rapidly expanding. As the industry moves more aggressively toward continuous manufacturing modes, utilizing Process Analytical Technology (PAT) and Process Intensification (PI) concepts, the critical role of transport phenomena becomes elucidated. The ability to transfer energy, mass, and momentum with directed purposeful outcomes is a worthwhile endeavor in establishing higher production rates more economically. Furthermore, the ability to obtain desired drug properties, such as size, habit, and morphology, through novel manufacturing strategies permits unique formulation control for optimum delivery methodologies. Bottom-up processing to obtain nano-sized crystals is an excellent example. Formulation and delivery are intimately coupled in improving bio-efficacy at reduced loading and/or better controlled release capabilities, minimizing side affects and providing improved therapeutic interventions. Innovative nanotechnology applications, such as simultaneous targeting, imaging and delivery to tumors, are now possible through use of novel chaperones. Other examples include nanoparticles attachment to T-cells, release from novel hydrogel implants, and functionalized encapsulants. Difficult tasks such as drug delivery to the brain via the blood brain barrier and/or the cerebrospinal fluid are now easier to accomplish.

Two-compartment model of radioimmunotherapy delivered through cerebrospinal fluid

PURPOSE

Radioimmunotherapy (RIT) using (131)I-3F8 injected into cerebrospinal fluid (CSF) was a safe modality for the treatment of leptomeningeal metastases (JCO, 25:5465, 2007). A single-compartment pharmacokinetic model described previously (JNM 50:1324, 2009) showed good fitting to the CSF radioactivity data obtained from patients. We now describe a two-compartment model to account for the ventricular reservoir of (131)I-3F8 and to identify limiting factors that may impact therapeutic ratio.

METHODS

Each parameter was examined for its effects on (1) the area under the radioactivity concentration curve of the bound antibody (AUC[C(IAR)]), (2) that of the unbound antibody AUC[C(IA)], and (3) their therapeutic ratio (AUC[C(IAR)]/AUC[C(IA)]).

RESULTS

Data fitting showed that CSF kBq/ml data fitted well using the two-compartment model (R = 0.95 ± 0.03). Correlations were substantially better when compared to the one-compartment model (R = 0.92 ± 0.11 versus 0.77 ± 0.21, p = 0.005). In addition, we made the following new predictions: (1) Increasing immunoreactivity of (131)I-3F8 from 10% to 90% increased both (AUC[C(IAR)]) and therapeutic ratio ([AUC[C(IAR)]/AUC[C(IA)]] by 7.4 fold, (2) When extrapolated to the clinical setting, the model predicted that if (131)I-3F8 could be split into 4 doses of 1.4 mg each and given at ≥24 hours apart, an antibody affinity of K(D) of 4 × 10(-9) at 50% immunoreactivity were adequate in order to deliver ≥100 Gy to tumor cells while keeping normal CSF exposure to <10 Gy.

CONCLUSIONS

This model predicted that immunoreactivity, affinity and optimal scheduling of antibody injections were crucial in improving therapeutic index.