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. Author manuscript; available in PMC: 2020 Oct 1.
Published in final edited form as: J Thorac Oncol. 2019 Jun 11;14(10):1743–1752. doi: 10.1016/j.jtho.2019.05.032

PET Imaging of a [18F]-radiolabeled PARP Inhibitor Monitors the Therapeutic Efficacy of Talazoparib in Small Cell Lung Cancer Patient-Derived Xenografts

James Laird a,b,*, Benjamin H Lok a,c,*,#, Brandon Carney d,e, Susanne Kossatz d, Elisa de Stanchina f, Thomas Reiner d,g,h, John T Poirier a, Charles M Rudin a,i
PMCID: PMC6764879  NIHMSID: NIHMS1531484  PMID: 31195178

Abstract

Introduction

Inhibitors of poly-(ADP)-ribose polymerase (PARP) are promising therapeutics for small cell lung cancer (SCLC). We tested whether PARP inhibitor (PARPi) target engagement as measured by a radiolabeled PARP inhibitor ([18F]PARPi) has the potential to predict drug efficacy in vivo.

Methods

Tumor growth inhibition during daily talazoparib treatment was evaluated in mice engrafted with SCLC patient-derived xenografts to evaluate talazoparib efficacy at multiple doses. Mice were intravenously injected with [18F]PARPi radiotracer at multiple time points after single doses of oral talazoparib to quantitatively assess the extent to which talazoparib could reduce tumor radiotracer uptake and PET/CT activity. Tumors were harvested and tumor PAR level was measured by ELISA.

Results

A dose range of talazoparib with differential therapeutic efficacy was established, with significant delay in time to reach 1000 mm3 for tumors treated with 0.3 mg/kg (p=0.02) but not 0.1 mg/kg talazoparib. On PET/CT with [18F]PARPi, reduction in [18F]PARPi uptake after talazoparib dosing was consistent with talazoparib clearance, with reduction in PET activity attenuating over 24 hours. Talazoparib target engagement, measured by maximum tumor PET uptake, increased in a dose dependent manner (3.9% vs. 2.1% ID/g for 0.1 and 0.3 mg/kg at 3 hours post-talazoparib, p=0.003) and correlated with PARP enzymatic activity among individual tumors as measured by total tumor PAR (p=0.04, R=0.62 at 1 hour post-talazoparib).

Conclusions

PET imaging using [18F]PARPi has the potential to be a powerful tool in treatment monitoring by assessing PARP inhibitor target engagement in real-time.

Keywords: small cell lung cancer, PARP inhibitors, PET, talazoparib, drug target engagement

Introduction

Inhibitors of poly-(ADP)-ribose polymerase (PARP) 1, a nuclear protein involved in DNA damage detection and repair, have been shown to be effective in multiple types of cancer, including breast, ovarian, prostate, and small cell lung cancers1–6. In small cell lung cancer (SCLC), PARP inhibitors are effective in cell lines and xenografts as a single agent and when combined with chemotherapy or radiation7–10. Multiple clinical trials of PARP inhibitors for patients with SCLC have yielded promising results, including for talazoparib, one of the most potent PARP inhibitors in clinical use11–14.

A methodology for assessing in real time which patients have disease that can be effectively targeted by PARP inhibitors, with drug-target engagement and quantitative enzymatic inhibition, could focus treatment to those patients most likely to benefit from therapeutic administration of PARP inhibitors. Individual patient outcomes are likely to be affected by variability in both drug delivery to tumor and drug effectiveness once it reaches the tumor. For the latter, several potential biomarkers have been identified in SCLC, including SLFN11, ATM, and E-cadherin8,10,11,15,16. These promising predictive biomarkers inform whether a particular tumor is inherently sensitive to PARP inhibition; however, they do not account for differences in patient drug metabolism/elimination (pharmacokinetics), tumor vascularity, or tumor-specific drug efflux. It is possible that insufficient drug delivery is responsible for treatment failure in a subset of patients. In order to address this, many phase I/II trials include pharmacokinetic (PK, e.g. plasma drug concentration) and surrogate pharmacodynamics (PD) measurements (e.g. drug effect on peripheral blood monocytes)14,17–19. However, these PK and surrogate PD methods are indirect measures that may poorly correlate with intratumoral drug concentrations, target engagement, and target inhibition in treated patients.

A theoretical approach to intratumoral PD analysis could include multiple serial re-biopsies of each anatomic site of disease over time. Clearly this approach is not feasible, as it would involve excessive patient risk and resource utilization. We present an alternative strategy of measuring PARP inhibitor pharmacodynamics that has the potential to discriminate between effective and ineffective drug exposure in target tumors through non-invasive molecular imaging. In our previous work, we demonstrated that a radiolabeled PARP inhibitor, [18F]PARPi, can be visualized by positron emission tomography (PET) for real-time in vivo PARP inhibitor target engagement assessment20. Here, we demonstrate the translational relevance of this approach, showing that [18F]PARPi PET can measure talazoparib target engagement in patient-derived xenografts at a range of oral doses that might be expected in trials of talazoparib efficacy and can distinguish therapeutic and subtherapeutic doses of talazoparib.

Materials and Methods

Mice and Xenograft Models

The Memorial Sloan Kettering Institutional Animal Care and Use Committee approved all animal protocols for this work. Female NSG mice (NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ; The Jackson Laboratory) that were 6 to 8 weeks old at time of patient-derived xenograft (PDX) implantation were used for all PET imaging and in vivo talazoparib efficacy experiments. The PDX SCRX-Lu149 was isolated and passaged as previously described21,22. PDXs were injected in the right shoulder for PET/CT experiments and in the right flank during tumor growth inhibition experiments.

Talazoparib Preparation and Administration

Talazoparib was obtained from Selleck Chemicals and stored at −20°C until drug preparation as previously described8. Solutions of 0.1 mg/mL talazoparib in 10% dimethylacetamide, 5% polyethoxylated castor oil, and 85% PBS or vehicle were stored at 4°C for up to 2 weeks and diluted in PBS prior to oral gavage.

Tumor Growth Inhibition

Tumor volumes were calculated from manual caliper measurements using the formula volume = (xy2)/2 where x is the longest diameter and y is the perpendicular, tangential to skin surface. For all tumor growth inhibition experiments, mice were randomized at a tumor volume of approximately 125–150 mm3 and talazoparib or vehicle was administered daily on Monday to Friday, for 20 total doses. Mouse weights and tumor volumes were measured twice weekly until mice were euthanized at a tumor size of 1000–1500 mm3.

For experiments testing the combination of talazoparib and radiation, mice were treated with vehicle, 0.1 mg/kg, or 0.3 mg/kg talazoparib as above with the addition of four 2 Gy daily fractions of radiation on dosing days 2–5. The mice were anesthetized with ketamine and xylazine injection and tumors were irradiated with an X-Ray irradiator (XRAD 320, Precision X- Ray). Radiation was administered 3 hours after drug dosing. Custom lead cutouts were used to reduce dose to normal mouse tissue.

[18F]PET/CT Imaging

The [18F]PARPi radiotracer was synthesized as previously described20,23. Mice were administered 0.1, 0.2, or 0.3 mg/kg doses of talazoparib by oral gavage at 1, 3, 6, 18, or 24 hours prior to radiotracer injection. Intravenous radiotracer administration occurred two hours prior to imaging, with 150–300 μCi of [18F]PARPi in 100–200 μL 10% EtOH in 0.9% sterile saline injected. Mice were anesthetized by inhalation of 2% isofluorane prior to being positioned on the scanner bed. PET/CT images were captured using an Inveon PET/CT (Siemens). PET images were obtained for 5–15 minutes, depending on radiotracer activity, followed by CT scan. PET/CT images were analyzed using the Inveon Research Workplace Software (Siemens). Tumors were contoured using CT images and mean and maximum tumor PET activities were quantified and reported as percent of injected dose per gram (%ID/g) ± SD.

Biodistribution Studies

Immediately after imaging, mice were euthanized and dissected. Segments of tumor were flash frozen for ex vivo analysis or included as an organ in biodistribution studies. Post-imaging biodistribution studies were performed as previously described23. Briefly, mice were euthanized immediately after PET/CT imaging and major organs were collected and weighed; organ radiotracer activity was then quantified using the Wizard2 automatic γ-counter (PerkinElmer, Boston, MA).

ELISA

Three mice per time point were administered 0.2 mg/kg talazoparib by oral gavage and euthanized for flash frozen tumor tissue collection at 1, 3, 6, 18, and 24 hours after dosing. For comparison of 0.1 and 0.3 mg/kg doses, flash frozen tumor tissue was collected after animal PET/CT imaging, euthanasia, and dissection, as above. Tumors were lysed and PAR values were quantified using the PARP in vivo Pharmacodynamic Assay II Kit (Trevigen).

Statistical Analysis

For in vivo tumor growth delay, time to reach 1000 mm3 was compared using Kaplan-Meier analysis and the log-rank test, and averages were compared using the Mann-Whitney test. For PET imaging studies, comparisons between treated groups and untreated control were made using Welch’s ANOVA with Dunnett’s test to adjust for multiple comparisons; comparisons between 0.1 and 0.3 mg/kg talazoparib groups were made with the t-test and the Holm-Sidak method to correct for multiple comparisons. Correlation between ex vivo tumor PAR value and PET activity was analyzed at the one-hour time point by calculating Pearson’s correlation coefficient (R) with two-tailed p values. P values <0.05 were considered significant. All analyses were performed using GraphPad Prism 7.0a (GraphPad Software, Inc.).

Results

Therapeutic and subtherapeutic doses of talazoparib

In order to ultimately assess the ability of the radiotracer to assess PARP inhibitor pharmacokinetics and pharmacodynamics, we first determined a dose range of talazoparib that could reflect differential therapeutic sensitivities in a PDX model of SCLC. The dose range selected was based on our prior published work optimizing in vivo talazoparib dosing as a single agent and in combination with fractionated radiotherapy (RT) or temozolomide8,9. Mice with SCRX-Lu149 implanted on the right flank were randomized to receive daily doses of vehicle, 0.1 mg/kg talazoparib, or 0.3 mg/kg talazoparib by oral gavage over 20 doses. The dose of 0.1 mg/kg was not sufficient to inhibit tumor growth, while 0.3 mg/kg talazoparib significantly reduced tumor growth compared to control (p = 0.02; Figure 1A).

Figure 1:

Figure 1:

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Doses of 0.1 and 0.3 mg/kg talazoparib differ in efficacy as a single agent and in combination with radiation. A) Tumor growth curves and Kaplan-Meier plot of SCRX-Lu149 PDX treated with 20 doses (on Monday-Friday) of vehicle, 0.1 mg/kg, or 0.3 mg/kg talazoparib by oral gavage. Each line represents one mouse. A Kaplan-Meier event was defined as tumor volume reaching 1000 mm3. B) Tumor growth curves and Kaplan-Meier plots as above with the addition of four daily doses of 2 Gy radiation applied on days 2–5. Radiation was delivered 3 hours after talazoparib administration.

We further examined the combination of these doses of talazoparib with concurrent radiation given talazoparib’s radiosensitizing properties9. For RT, we applied the conventional clinical fraction size of 2 Gy per daily fraction. Total cumulative RT dose delivered was chosen to balance combination effect detection and to mitigate potential toxicity based on prior experience9. These doses and their combination were used as a benchmark to determine the translational utility of the[18F]PARPi radiotracer. Accordingly, mice were treated with 20 daily doses of vehicle, 0.1 mg/kg talazoparib, or 0.3 mg/kg talazoparib with four 2 Gy fractions of radiation administered 3 hours after talazoparib gavage in each group on days 2–5 of drug dosing. Similar to its efficacy as a single agent, 0.1 mg/kg talazoparib did not produce statistically significant tumor growth inhibition in combination with radiation, while tumor growth inhibition for a dose of 0.3 mg/kg talazoparib was statistically different (p = 0.04; Figure 1B).

[18F]PARPi PET imaging and biodistribution

Having established a dose range of talazoparib that included therapeutic and subtherapeutic drug doses, we began exploring the ability of our radiotracer to quantitatively assess talazoparib target engagement. We first confirmed that the18Flabeled PARP inhibitor ([18F]PARPi), which is structurally similar to olaparib (Figure 2A), allowed imaging of the PARP1-expressing SCLC PDX SCRX-Lu14923. PET/CT images of mice two hours after injection of [18F]PARPi showed PET activity localized to the tumor, with clear discrimination from surrounding normal tissue (Figure 2B). This observation was also seen during quantification of organ radioactivity by γ-counting. Tumor was among the tissues with the highest radioactivity per gram (1.37 ± 0.15 %ID/g), significantly higher than surrounding lung (0.24 ± 0.05 %ID/g, p = 0.007), bone (0.27 ± 0.05 %ID/g, p = 0.007), and muscle tissue (0.24 ± 0.15 %ID/g, p < 0.002; Supplemental Figure S1).

Figure 2:

Figure 2:

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[18F]PARPi PET/CT Experimental Design. A) Molecular structures of olaparib, talazoparib, and an 18F-labeled PARP inhibitor ([18F]PARPi) are demonstrated. The [18F]PARPi is formed by conjugating a 4-[18F]fluorobenzoic acid group to a scaffold of the small molecule olaparib. B) When [18F]PARPi is present without the competitive binding of talazoparib, it binds to the PARP1 enzyme, allowing visualization of the high PARP expressing SCLC PDX SCRX-Lu149. Tumor is marked with a “T” on PET images. Tumors are implanted in the right shoulder to prevent signal interference from the gut, as the [18F]PARPi radiotracer is eliminated by the hepatobiliary system and subsequently has increased activity in the intestines. C) Pre-treatment with 0.2 mg/kg talazoparib by oral gavage competitively blocks the binding of [18F]PARPi and results in lower activity. D) An experimental timeline demonstrates the method used for talazoparib pre-treatment and PET scanning. Talazoparib was orally dosed at 0.1, 0.2, or 0.3 mg/kg at various time points prior to intravenous injection of [18F]PARPi. Two hours after radiotracer injection, PET/CT imaging was performed, and mouse dissection was performed immediately after imaging.

We hypothesized that an oral dose of talazoparib would reduce intratumoral [18F]PARPi PET activity when administered prior to imaging due to the radiotracer being blocked from its binding site by talazoparib (Figure 2C). The timing of oral talazoparib dosing relative to imaging for all PET imaging experiments is illustrated in Figure 2D. When mice were administered 0.2 mg/kg talazoparib by oral gavage one hour before intravenous injection of [18F]PARPi, mean tumor PET activity (71.1% relative reduction in %ID/g; p = 0.04) and maximum tumor PET activity (59.4% reduction in %ID/g, p = 0.03) were both significantly reduced compared to control (Figures 3A–C). Tumor radioactivity by γ-count was reduced by 54.9% in mice pre-treated with talazoparib (p = 0.01, Figure 3D). Similar reductions in organ radioactivity ranging from 40–75% were noted in normal lung, stomach, kidney, bone, and blood (Supplemental Figure S2). The spleen was the organ with the greatest percent reduction in radioactivity at 89.1%. As expected, large intestine and liver exhibited 18F activity due to [18F]PARPi metabolism and elimination, with low or non-significant reductions in radioactivity with talazoparib pre-treatment. This demonstrates the concordance of [18F]PARPi with the known routes of metabolism for PARP inhibitors in clinical use24.

Figure 3:

Figure 3:

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[18F]PARPi models talazoparib pharmacokinetics at a single dose. A) A dose of talazoparib 0.2 mg/kg was delivered by oral gavage at varying time intervals prior to radiotracer injection and PET/CT imaging. Maximum intensity projection images taken from mice treated are displayed, with subcutaneous SCRX-Lu149 PDX tumors denoted with “T”. B) Maximum and C) mean PET activities in each tumor were quantified, with the greatest reduction in PET activity at 1 to 3 hours post-talazoparib oral gavage. Each dot represents one mouse/PDX tumor. D) γ-counts of the tumors harvested immediately after imaging show similar trend in radiotracer activity.

[18F]PARPi PET models target engagement by orally administered talazoparib

To explore the utility of [18F]PARPi PET as a marker of intratumoral PARP inhibitor concentration at therapeutically relevant oral talazoparib doses, we then determined if [18F]PARPi PET was able to model intratumoral talazoparib pharmacokinetics after drug dosing. In addition to the 1-hour timepoint above, mice were administered 0.2 mg/kg talazoparib by oral gavage at time points 3 to 24 hours before radiotracer injection and imaging (Figure 2D; Figure 3A). For both the maximum (Figure 3B) and mean (Figure 3C) [18F]PARPi uptake values, blocking of intratumoral radiotracer binding was greatest at 1 to 3 hours after talazoparib administration. As expected, talazoparib blocking decreased over time with progressive increase in the PET signal at subsequent time points. Using the orthogonal measure of radiotracer uptake in the tumor, we confirmed similar differences by γ-counting of the tumor immediately after PET imaging (Figure 3D).

Therapeutic and subtherapeutic oral doses of talazoparib can be distinguished by [18F]PARPi

Having established that [18F]PARPi PET models the pharmacokinetics of a single dose of talazoparib, we then postulated that [18F]PARPi PET could differentiate between 0.3 mg/kg talazoparib, which was therapeutic against SCRX-Lu149, and the subtherapeutic dose of 0.1 mg/kg talazoparib. Maximum tumor PET activity was significantly reduced compared to untreated control following administration of 0.3 mg/kg talazoparib at 1 hour (p = 0.04) and 3 hours (p = 0.05) but not following 0.1 mg/kg talazoparib, with 0.3 mg/kg talazoparib having significantly reduced maximum tumor PET activity compared to the 0.1 mg/kg dose at 3 hours (p = 0.003, Figure 4A, B). Similarly, mean PET activity following 0.3 mg/kg talazoparib was consistently lower than that following 0.1 mg/kg; however these differences did not reach statistical significance when adjusting for multiple comparisons (Figure 4C). Similar trends were noted on quantification of post-dissection γ-counting (Figure 4D). On biodistribution analysis, there were no significant differences in radioactivity of non-tumor organs between the 0.1 and 0.3 mg/kg groups (Supplementary Figure S3).

Figure 4:

Figure 4:

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[18F]PARPi discriminates between high and low doses of talazoparib and correlates with PARP inhibitor activity. A) Doses of 0.1 mg/kg and 0.3 mg/kg talazoparib were administered to mice by oral gavage between 1 and 24 hours prior to [18F]PARPi radiotracer injection and PET/CT imaging. Maximum intensity projection images of mice are displayed, with SCRX-Lu149 PDX tumors denoted with a “T”. B) Maximum and C) mean PET activities in each tumor were quantified at each time point and dose. Each dot represents one mouse/PDX tumor for subpanels B-G. Significant differences in both mean and maximum PET activity between 0.1 mg/kg and 0.3 mg/kg were noted at the 3 hour time point. D) γ-counts of the tumors, harvested immediately after PET/CT imaging, show similar results as PET/CT imaging. E) Samples of tumors from the imaged mice were flash frozen immediately after PET/CT imaging, followed by quantification of PAR polymers, the product of the PARP enzyme, by ELISA. F) Maximum and G) mean PET signals are plotted against tumor PAR levels as quantified by ELISA for untreated control mice and mice treated with talazoparib at the one hour time point. Maximum PET signal correlated with tumor PAR and mean PET signal trended towards correlation (p values and Pearson’s correlation coefficient (R) displayed on graph). Lines of best fit are displayed with 95% confidence intervals (dotted lines).

[18F]PARPi PET correlates with total tumor PAR

PARP1 enzyme activity leads to the accumulation of poly-(ADP) ribose (PAR) chains, and measurement of the reduction of PAR chain accumulation after PARP inhibition is a pharmacodynamic readout of PARP inhibitor activity13. Therefore, we assessed the ability of talazoparib to reduce total PAR in 29 tumors harvested from mice after oral administration of 0.1 or 0.3 mg/kg talazoparib and PET/CT imaging. Using ELISA to measure flash frozen tumor lysate PAR concentration, variations in talazoparib dose and timing had similar effects on PAR concentration as compared to [18F]PARPi PET activity (Figure 4E). At the one hour time point, there was a statistically significant correlation between the ex vivo tumor PAR and maximum PET activity (p = 0.04, R = 0.62; Figure 4F), and a similar trend that did not meet statistical significance between ex vivo tumor PAR and mean PET activity (p = 0.22, R = 0.40; Figure 4G).

Discussion

In this study, we demonstrate that PET imaging with [18F]PARPi has the potential to measure intratumoral talazoparib target engagement in PDXs. When SCLC PDX-implanted mice are treated with a single therapeutic dose of the PARP inhibitor talazoparib and then imaged with [18F]PARPi PET, PET signal is decreased by varying degrees according to talazoparib dose and timing of administration. The decrease in PET signal appears to model PARP inhibitor pharmacokinetics, correlates with PARP inhibitor pharmacodynamics as measured by tumor PAR levels, and differs significantly between therapeutic and subtherapeutic doses of talazoparib.

In SCLC, which carries a particularly poor prognosis and has had few changes to its treatment paradigm in the last several decades, PARP inhibitors represent a promising investigational treatment option. In a phase II trial comparing the PARP inhibitor veliparib and temozolomide (TMZ) with TMZ monotherapy in patients with relapsed SCLC, veliparib achieved a significantly higher objective response rate compared to placebo (39% vs. 14%, p = 0.016) 11. Another randomized phase II study compared cisplatin and etoposide in combination with veliparib or placebo for first-line treatment of extensive-stage SCLC and demonstrated improved progression-free survival with veliparib (stratified hazard ratio 0.63; 1-sided p-value 0.01)12. Multiple other ongoing clinical trials are currently exploring how PARP inhibition may be added to other therapies in SCLC, including radiation (i.e. NCT03672773, NCT02446704, NCT03532880). However, development of biomarkers to assess target engagement and extrapolate pharmacodynamic properties could help focus clinical testing of PARP inhibitors as a novel therapeutic for SCLC in patient subsets most likely to benefit from these targeted agents.

Current biomarkers of PARPi therapy

One important aspect of treatment is to determine whether a drug is reaching its target and acting as intended. Ideally, PARP inhibitor efficacy would be monitored by direct measurement of PAR and γH2AX formation in short interval post-treatment tumor biopsies25. However, this would cause patients significant discomfort and expose them to unnecessary risk. Biosampling of surrogate tissues has therefore been performed to monitor pharmacodynamics in early phase clinical trials; these include PAR measurement in peripheral blood mononuclear cells (PBMCs) and in plucked hair cells 26–29. Unfortunately, these and other biosampling methods in use do not provide a direct measurement of PARP inhibitor activity in the drug’s primary target, the tumor itself. The tumor microenvironment differs substantially from these surrogate tissues in oncotic and hydrostatic pressure, microvessel composition, and other factors that can influence local drug delivery, export, and target engagement. PET imaging of a tumor using a radiolabeled derivative of a PARP inhibitor circumvents these limitations and has potential to be a non-invasive biomarker of a PARP inhibitor target engagement in the tumor itself. Advantages of this approach include safety, decreased sampling bias, and ability to simultaneously visualize multiple tumor sites with internal reference to normal tissues.

PARP PET radiotracers

Multiple PET radiotracers to image PARP have been described in the literature30. Our radiotracer, [18F]PARPi, as well as 18F-BO, 124I-PARPi, and 18F-olaparib are structurally similar to olaparib while 18F-FTT, 125I-KX1, and 125I-KX-02–019 are similar to rucaparib31–37. These radiotracers have been shown to measure PARP expression in vivo, with one recent study by Makvandi et al. demonstrating a correlation with PARP expression in humans38. However, while experimentally generated PARP1 knockout cells are resistant to PARP inhibition, it is unclear if the differences in endogenous PARP expression in patient tumors correlate with PARP inhibitor efficacy39,40. As discussed in Thomas et al., PARP1 deficiency, which yields resistance to PARP inhibitors, is rare in cancer cell line databases and would not be expected to be present in human tumors40. In addition, for SCLC cell lines, PARP expression does not correlate with PARP inhibitor efficacy8. Therefore, an imaging method that only quantifies the amount of tumor PARP expression, as opposed to assessing the dynamics of target engagement and inhibition, may be unlikely to predict PARP inhibitor efficacy. Continued work to investigate the optimal diagnostic, prognostic and predictive applications of these novel PARP molecular imaging agents for our patients are warranted.

Current and future development of [18F]PARPi

Here, we describe a novel method of imaging that is distinct from those described above, quantifying target engagement of a clinical PARP inhibitor via the decrease in radiotracer uptake rather than PARP expression by radiotracer uptake in the absence of PARP inhibitor. Our group has previously shown the potential of this approach by modeling olaparib and talazoparib pharmacokinetics at various doses and time points in the SCLC PDX JHU-LX4820. In the current study, we substantially develop these findings and show that radiotracer uptake with this approach correlates with PARP inhibitor pharmacodynamics through tumor PAR measurements. We also demonstrate that this approach can differentiate therapeutic and subtherapeutic talazoparib doses. Our data suggests that measuring the decrease in tracer uptake after administration of a PARP inhibitor may be a viable method of determining PARP inhibitor delivery to the tumor and ultimately be useful in treatment monitoring.

With respect to its clinical-translational potential, imaging with [18F]PARPi would be expected to be safe in humans as [18F]PARPi has inhibitory activity similar to olaparib but the dose required for imaging is approximately 60,000 times less than the daily treatment dose of olaparib20. We are now exploring the utility of [18F]PARPi in human trials for head and neck cancer patients (NCT03631017). Reassuringly, other PARP inhibitor radiotracers (18F-FTT, and [125I]KX1) have been safely tested in patients, with successful imaging of pancreatic, hepatic, and ovarian tumors, including in areas not amenable to [18F]FDG imaging such as near the bladder41.

The application of [18F]PARPi imaging in the context of PARPi combinations with cytotoxic drugs currently under clinical investigation, including platinum/etoposide or temozolomide, is yet to be determined. We hypothesize that this novel approach with PARPi pharmacodynamic assessment by [18F]PARPi imaging may be a useful tool to determine the contribution of PARPi to a combination regimen. Ideally, a method that incorporates intrinsic tumor sensitivity to a drug combination and real-time pharmacodynamic assessment of each therapeutic agent would be necessary to fully resolve these considerations. Nonetheless, we demonstrate an important step towards the novel translational application of [18F]PARPi imaging for PARPi target engagement in both single agent and combination contexts.

To test the performance of this approach, we consistently focused on analysis of a single SCLC PDX model for all in vivo experimentation. The homogenous genetic background of both the host animals and tumor allowed us to better address our central research question with respect to multiple relevant dose and time-dependent endpoints. Caution should be extended with respect to whether these findings will translate to all SCLC tumors or other tumor histologies. Furthermore, the use of the same mouse strain background limits the variability in drug pharmacokinetics and tumor delivery that could be detected. Further experiments with a diverse population of models and tumors are warranted. In this setting, it may be possible to see differences in tracer uptake within a single treatment dose that vary with host and tumor differences and correspond with drug pharmacodynamics and efficacy.

Conclusion

In summary, molecular imaging with a PARP inhibitor radiotracer, [18F]PARPi, can quantify the amount of PARP inhibitor target engagement and informs PARP inhibitor efficacy in SCLC PDX. This approach represents a powerful potential tool to monitor PARP inhibitor treatment in real-time. In an era where treatments are increasingly personalized by a patient’s tumor genotype, molecular imaging may represent a complementary method of assessing and individualizing treatment pharmacodynamics to improve patient outcomes.

Supplementary Material

1

Acknowledgements

The authors thank Nisarg Shah, Ahmad Sadique, and Patrick Donabedian for assistance with radiotracer synthesis, PET imaging, and mouse dissection; Craig McCarthy and the members of the Antitumor Assessment Core Facility for their technical assistance; Ronglai Shen for biostatistical consultation; Viola Allaj for assistance with ELISA; and all members of the Rudin lab for their guidance and input.

Financial Support: The work of the authors is supported by funding from the Conquer Cancer Foundation of ASCO, Lung Cancer Research Foundation, Radiological Society of North America, International Association for the Study of Lung Cancer, and the Clinical and Translational Science Center at Weill Cornell Medical Center and MSKCC (UL1TR00457; to B.H.L.), the National Institutes of Health (P30 CA008748, R01 CA197936 to C.M.R., R01 CA204441 to T.R. and K99 CA218875 to S.K.) and the MSK Cancer Center Support Grant/Core Grant (U54 OD020355–01). The authors thank the Tow Foundation and MSK’s Center for Molecular Imaging & Nanotechnology, Imaging and Radiation Sciences Program, and the Molecularly Targeted Intraoperative Imaging Fund. The funding sources had no involvement in study design, data collection and analysis, writing of the report, or the decision to submit this article for publication.

Footnotes

Conflict of Interest: S.K. and T.R. are shareholders of Summit Biomedical Imaging, LLC. S.K. and T.R. are co-inventors on filed U.S. patent (WO2016164771) held by MSK that covers methods of use for PARPi-FL. T.R. is a co-inventor on U.S. patent (WO2012074840) held by the General Hospital Corporation that covers the composition of PARPi-FL. B.C. and T.R. are co-inventors on the U.S. patent (US20170266327A1) held by MSK that covers methods for the synthesis of [18F]PARPi. C.M.R. has consulted regarding oncology drug development for AbbVie, Amgen, Ascentage, AstraZeneca, BMS, Celgene, Daiichi Sankyo, Genentech/Roche, Ipsen, Loxo, and Pharmar, is on the scientific advisory boards of Bridge, Elucida and Harpoon, and receives research funding from Daiichi Sankyo. J.L., E.S., J.P., and B.L. declare no conflicts of interest.

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