Phase I Clinical Trial of High Doses of Seleno-L-methionine in Combination with Axitinib in Patients with Previously Treated Metastatic Clear Cell Renal Cell Carcinoma

Abstract

Background:

Data in clear cell renal cell carcinoma (ccRCC) xenografts defined the seleno-Lmethionine (SLM) dose and the plasma selenium concentrations associated with the enhancement of HIF1α/2α degradation, stabilization of tumor vasculature, enhanced drug delivery, and efficacy of axitinib. The data provided the rationale for the development of this phase I clinical trial of SLM and axitinib in advanced or metastatic relapsed ccRCC.

Patients and Methods:

Patients were ≥18 years with histologically and radiologically confirmed advanced or metastatic ccRCC who had received at least one prior systemic therapy, which could include axitinib (last dose ≥6 months prior to enrollment). Escalating dose levels of SLM (2500, 3000 and 4000 μg) were administered orally twice daily for 14 days, then once daily concurrently with axitinib 5 mg twice daily using a 3+3 design in Phase I. Patients were treated at the 4000 μg dose level in the expansion cohort to obtain preliminary estimates of efficacy.

Results:

No dose limiting toxicities occurred at the 4000 μg SLM dose level. Among the 27 patients treated with 4000 μg of SLM, the overall response rate was 55.6%, median duration of response was 18.4 months, median progression-free survival was 14.8 months, and median overall survival was 19.6 months. Preliminary results have shown that plasma selenium concentrations, inhibition of TGF-β1 and stabilization of tumor vasculature by SLM are time dependent.

Conclusions:

SLM (4000 μg) in sequential combination with axitinib is well tolerated with encouraging efficacy.

Introduction

There are approximately 82,000 new cases and almost 15,000 deaths from renal cell carcinoma (RCC) each year in the U.S(1), most commonly clear cell renal cell carcinoma (ccRCC), which comprises 75–85% of all kidney cancers (2). ccRCC is characterized by deficiency or mutation of Von Hippel-Lindau (VHL) and polybromo-1 (PBRM-1) tumor suppressor genes (3), and stable expression of hypoxia-inducible factor-1α and −2α (HIFs) (4,5). The altered expression of these biomarkers leads to transcription of a variety of hypoxia-responsive genes including vascular endothelial growth factor (VEGF), platelet-derived growth factor, and fibroblast growth factor. This contributes to the unstable tumor microenvironment and vasculature, increases tumor angiogenesis, immune evasion, and drug resistance.

Axitinib is an oral, second-generation selective inhibitor of the VEGF receptor (VEGFR-1, −2, and −3), and is approved as a single agent for second line and beyond therapy in patients with advanced ccRCC with an objective response rate (ORR) of 19.4%, median progression-free survival (PFS) of 8.3 months and median overall survival (OS) of 20 months (6,7).

Our preclinical studies demonstrate that therapeutic and nontoxic doses of organic Semethylselenocysteine (MSC) and seleno-L-methionine (SLM) are potent inhibitors of both HIFs(4) and Transforming growth factor-beta (TGF-β1), which is broadly dysregulated in ccRCC tumors ((8), paper under review). These effects were associated with significant antitumor activity of multiple anticancer drugs in several xenograft models including ccRCC(4,9–14). MSC and SLM do not inhibit HIF protein synthesis; rather, they facilitate degradation through a unique VHL-independent and propyl hydroxylase-2-dependent pathway (4). In animal models, MSC and SLM have demonstrated angiogenic effects resulting in maturation of tumor vasculature, enhanced drug delivery to tumor tissues (12–14) and demonstrated synergy with anticancer therapy that was selenium dose- and schedule-dependent, whereas sequential administration of selenium followed by targeted or chemotherapy resulted in better anti-tumor activity compared to concurrent administration (9,15). On this basis, we hypothesized that simultaneous and concurrent inhibition of specific biomarkers altered in most patients with ccRCC by therapeutic doses of SLM would synergistically enhance the antitumor activity of axitinib and may also reduce incidence and severity of adverse events. To test this hypothesis, we conducted a phase I clinical trial of escalating doses of SLM in sequential combination with standard of care axitinib in patients with advanced or metastatic ccRCC who were previously treated and relapsed on at least one prior line of therapy.

Materials and Methods

Study Design

This was a Phase 1 open-label, single-arm, dose escalation and expansion study designed to evaluate the safety and tolerability and to estimate the preliminary anti-tumor activity of SLM in combination with axitinib in the treatment of advanced or metastatic ccRCC (NCT02535533).

The initial trial design followed a 3+3 approach, with the highest pre-specified planned dose of SLM set at 4,000 μg. This dose was anticipated to achieve plasma concentrations comparable to those found to be synergistic with axitinib in xenograft models. Escalating dose levels of SLM (2500, 3000 and 4000 μg) were administered orally twice daily for 14 days during a run-in period. After which, SLM once daily and axitinib 5 mg twice daily were administered orally. Axitinib could be titrated according to the standard of care in a 28-day cycle. Selenium dose interruption was permitted. Dose delay for more than 4 weeks led to study discontinuation. Applicable to the first cycle, dose-limiting toxicities (DLTs) were defined as treatment-related grade 3 or higher non-hematological or grade 4 hematological adverse events (AE), grade 4 nausea, vomiting, diarrhea, or electrolyte imbalances or grade 3 nausea, vomiting, diarrhea and electrolyte imbalances lasting greater than 48 hours despite optimal medical intervention. Adverse events were graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI-CTCAE) Version 4.03 (16). In the expansion cohort, patients were treated at the MTD.

Patients were followed clinically every 2 weeks for the first 2 cycles and thereafter every 4 weeks. Disease assessments were every 8 weeks using a CT scan of the chest, abdomen, and pelvis along with a bone scan at the provider’s discretion. Radiologic response was determined using the Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1.

All patients provided written informed consent and were treated until disease progression, unacceptable toxicity or meeting other stopping criteria. Patients removed from study treatment for unacceptable AEs were monitored until resolution or stabilization. The study was approved by the University of Iowa Institutional Review Board (IRB: 201507716) and conducted in accordance with the ethics principles of the Declaration of Helsinki and the International Council of Harmonization Guidelines on Good Clinical Practice.

Patients

Eligible patients were ≥18 years with histologically and radiologically confirmed advanced or metastatic ccRCC and had received at least one prior systemic therapy. Prior therapy could include axitinib, though the last dose must have been at least 6 months prior to enrollment. Other key inclusion criteria included measurable disease by RECIST 1.1, adequate bone marrow, liver, and renal functions, and an Eastern Cooperative Oncology Group (ECOG) performance status of 0–2. Key exclusion criteria included symptomatic or untreated central nervous system metastases, uncontrolled hypertension (defined as >150/90 mm despite adequate medications), concomitant malignancy and other comorbidities including myocardial infarction, uncontrolled angina, congestive heart failure, or cerebrovascular accident within the previous 6 months.

Plasma and Tumor Concentration (Selenium and TGF-β1)

Total plasma selenium concentrations were quantitated on day 1 prior to SLM, and on day 14 and at cycles 2–7, 2 hours post-treatment. Selenium concentrations were determined using a NexION 350D Inductively Coupled Plasma Mass Spectrometer (ICP-MS; PerkinElmer, Waltham, MA). Analysis was performed on plasma following a 50-fold dilution using the sample diluent. Samples were diluted by adding 50 μL plasma and 50 μL of 3% HCl to 2.4 mL of sample diluent in a 12×75 mm polypropylene tube. Mixing was accomplished using a vortex mixer. Selected calibration standards were analyzed as samples at the beginning and end of the run to validate the accuracy of the calibration. External proficiency test materials were analyzed as a second-source check of the calibration standards.

TGF-β1 concentrations were determined using ELISA kits. The standard curve for TGF-β1 (Catalog # DB100C; R&D systems, Minneapolis, MN) was created, and patient plasma samples were quantified for TGF-β1 according to the instructions provided with each respective kit. All samples were assayed in triplicate.

Twenty-five mg of tumor tissue was finely minced using a sharp blade, placed into 2 ml Eppendorf tubes, and 10 μl of dasatinib-d8 at a concentration of 10 ng/ml was added as an internal standard. The mixture was then vortexed for 30 seconds. Subsequently, 990 μl of acetonitrile was added, and the solution was vortexed again for 1–2 minutes. The tubes were then placed in an ice bath for 30 minutes. Tubes were centrifuged at 20,000 g for 10 minutes and the acetonitrile was evaporated using liquid nitrogen. Samples were then reconstituted by adding 100 μl of a 50:50 mixture of 0.1% trifluoroacetic acid and acetonitrile. The tubes were vortexed for 1–2 minutes and then sonicated for 15 minutes. Tubes were centrifuged at 20,000 g for 5 minutes and 100 μl of the clear supernatant was transferred to HPLC vials to measure the concentration at λ= 340.

Dual Energy Computed Tomography

Dual energy computed tomography (DECT) of the chest, abdomen, and pelvis was conducted to capture tumor vascular changes prior to study treatment initiation, on day 14, and after 2 cycles (2 months) of daily SLM in combination with axitinib. All DECT scans were performed on a SOMATOM Force DECT system (Siemens Healthineers, Germany) with caudocranial scan direction. A dedicated DECT analysis software program (Siemens Syngo.via) was used with consistent placement of regions of interest within the tumors, normal lung tissue, and normal liver tissue across the three acquisition timepoints to measure iodine densities.

Statistical Considerations

The primary objective of Phase I was to establish the MTD of SLM in combination with axitinib using a 3+3 design. The MTD was defined as the highest dose level in which at most 1 out of 6 patients experienced a DLT. The primary objective of the expansion cohort was to obtain preliminary evidence of the anti-tumor activity of SLM in combination with axitinib to inform a subsequent Phase II trial.

The incidence of treatment-emergent adverse events was summarized by type of adverse event and grade with the most severe grade per patient being reported. The ORR was defined as the proportion of patients with a confirmed complete response (CR) or partial response (PR). Duration of response (DOR) was defined as the time from first documentation of response to progression. PFS was defined as the time from study treatment initiation to the date of first documentation of disease progression or death due to any cause. Otherwise, patients were censored at date of last radiographic assessment. OS was defined as the time from study treatment initiation to death due to any cause. Patients still alive were censored at the last date known to be alive. Survival probabilities were estimated and plotted using the Kaplan-Meier method. Estimates along with 95% pointwise confidence intervals are reported. Mixed effects regression models were used to evaluate changes in plasma TGF-β1 levels, plasma and tumor SLM and axitinib concentrations, and iodine densities in lesions over time. Random effects were included to account for the longitudinally correlated nature of repeat assessments within a patient. Mean estimates and 95% confidence intervals are reported. Spearman rank-order correlations were calculated to quantify the association between plasma and tumor SLM and axitinib concentrations, and plasma TGF-β1 levels. All statistical testing of correlative endpoints was two-sided and assessed for significance at the 5% level.

Data availability statement

The data generated in this study are not publicly available due to patient privacy requirements but can be made available upon reasonable request to the corresponding author.

Results

A total of 35 patients started the study treatment between March 2016 and August 2021 and were included in the reported safety analysis. In Phase I, 15 patients were treated at SLM doses of 2,500 μg (n=3), 3,000 μg (n=5) and 4,000 μg (n=7). The Data Safety and Monitoring Committee (DSMC) determined 3 patients were not DLT evaluable at SLM doses of 3,000 μg (n=2) and 4,000 μg (n=1). The 27 patients treated at the MTD (Phase I: 7, Expansion: 20) were included in the efficacy analysis.

Safety

The 35 patients included in the safety analysis were predominately male (30/35), non-Hispanic White (34/35) with a median age of 60. A total of 32 (91.4%) patients experienced at least one treatment-related AE, and 19 (54.3%) patients experienced a serious adverse event in which 4 were related to treatment. The most common treatment-related AEs seen with the combination were fatigue (77.1%), diarrhea (68.6%), anorexia (60.0%), weight loss (57.1%), nausea (51.4%) and hypertension (48.6%, Table 1). The most common grade (G) 3 toxicities were hypertension (34.3%), weight loss (20.0%), and fatigue (17.1%). The frequency of treatment-emergent AEs is listed in Supplemental Table 1. During the first 14 days of SLM mono-therapy, one (2.9%) patient experienced G3 lipase elevation, and the most common AEs seen were anorexia (17.1%, 4 G1, 2 G2), nausea (11.4%, 4 G1) and fatigue (8.6%, 3 G1). No difference in AEs were observed with the 3 different dose levels of SLM.

Table 1.

Adverse events possibly, probably or definitely related to study treatment occurring in ≥20% of patients in the safety population (N=35).

		Grade
Toxicity	Total	1–2	3
Fatigue	27 (77.1%)	21 (60.0%)	6 (17.1%)
Diarrhea	24 (68.6%)	21 (60.0%)	3 (8.6%)
Anorexia	21 (60.0%)	17 (48.6%)	4 (11.4%)
Weight loss	20 (57.1%)	13 (37.1%)	7 (20.0%)
Nausea	18 (51.4%)	18 (51.4%)	0 (0%)
Hypertension	17 (48.6%)	5 (14.3%)	12 (34.3%)
Hoarseness	16 (45.7%)	16 (45.7%)	0 (0%)
Palmar-plantar erythrodysesthesia syndrome	14 (40.0%)	13 (37.1%)	1 (2.9%)
Dyspnea	12 (34.3%)	12 (34.3%)	0 (0%)
Cough	11 (31.4%)	11 (31.4%)	0 (0%)
Proteinuria	11 (31.4%)	7 (20.0%)	4 (11.4%)
Dysgeusia	9 (25.7%)	9 (25.7%)	0 (0%)
Vomiting	9 (25.7%)	9 (25.7%)	0 (0%)
Generalized muscle weakness	8 (22.9%)	6 (17.1%)	2 (5.7%)
Constipation	7 (20.0%)	7 (20.0%)	0 (0%)
Mucositis oral	7 (20.0%)	6 (17.1%)	1 (2.9%)

MTD Determination

An initial cohort of 3 patients was treated at a SLM dose of 4,000 μg. While no DLTs were evidenced in this initial cohort, collectively with the observed preliminary efficacy and side effects, the SLM dose was de-escalated to 2,500 μg per DSMC’s recommendation. In the next 2 cohorts of 3 patients treated with 2,500 and 3,000 μg of SLM, no DLTs were evidenced and SLM dose was escalated back to 4,000 μg. Of the 3 DLT evaluable patients treated in the second cohort with 4,000 μg of SLM, no DLTs were evidenced. The 4,000 μg SLM dose in combination with 5 mg of axitinib was carried forward to the expansion cohort.

Efficacy

Among the 27 patients treated with 4,000 μg of SLM, the median age was 61 years (range: 39–76) and 85.2% were male (Table 2). Most patients had an ECOG performance status of 0(77.8%) and were intermediate risk (63.0%) per the International Metastatic Renal Cell Carcinoma Database Consortium (IMDC). Three (11.1%) patients had tumor with sarcomatoid differentiation. Patients had a median of 2 (range: 1–4) prior lines of systemic therapy for metastatic disease and 5 (18.5%) patients had received ≥3 prior lines of therapy. Two patients had prior treatment with axitinib in other clinical trial settings.

Table 2.

Patient demographics and baseline characteristics of the efficacy population (N=27).

Variable	N=27
Age, median (min-max)	61 (39–76)
Sex
Female	4 (14.8%)
Male	23 (85.2%)
ECOG Performance Status
0	21 (77.8%)
1	6 (22.2%)
Race or Ethnic Group
White	26 (96%)
Black	1 (4%)
IMDC Risk Group
Favorable	6 (22.2%)
Intermediate	17 (63.0%)
Poor	4 (14.8%)
Sarcomatoid features	3 (11.1%)
Prior systemic therapies, median (min-max)	2 (1–4)
Prior systemic therapies
1	13 (48.1%)
2	9 (33.3%)
≥3	5 (18.5%)
Prior anticancer therapiesa
Ipilimumab + Nivolumab	11 (40.7%)
Nivolumab	6 (22.2%)
Pazopanib	6 (22.2%)
Cabozantinib	6 (22.2%)
Sunitinib	5 (18.5%)
Durvalumab + Guadecitabine	5 (18.5%)
Everolimus	3 (11.1%)
Sunitinib + AGS-003	2 (7.4%)
Axitinib + TRC-105	1 (3.7%)
Axitinib + X4P	1 (3.7%)
Interleukin-2	1 (3.7%)

Abbreviations: IMDC, International Metastatic RCC Database Consortium.

^aPatients received ≥1 prior anticancer therapies including axitinib.

The median duration of treatment was 9.9 months (range: 0.4–43.3). Reasons for treatment discontinuation included disease progression (n=18), side effects (n=4), patient withdrawal (n=2), alternative therapy (n=2) and other comorbidities (n=1). Two (7.4%) patients achieved a CR, and 13 (48.1%) patients achieved a PR resulting in an ORR of 55.6% (95% CI: 35.3–74.5%, Figure 1A). Of note, one patient, who had necrotic lesions and achieved a partial response to study treatment per RECIST 1.1 criteria, underwent metastectomy with final pathology suggesting necrotic nodules with no tumor cells identified. The median DOR was 18.4 months (95% CI: 7.4-not reached). An ongoing CR and PR was evidenced in 1 and 5 patients at last disease evaluation, respectively. Patients’ treatment duration and disease status over time is depicted in Figure 1B. The change in the sum of target disease in relation to other efficacy endpoints and over time is shown in Figures 1C–1D. The two patients with prior axitinib exposure experienced disease progression within the first three months of enrollment in the clinical trial. One of these patients developed a brain lesion in the context of a seizure that occurred after starting the SLM loading dose. This patient had no other neurological symptoms during screening to warrant a baseline brain MRI.

Figure 1:

(A) Investigator-assessed best overall response. (B) Treatment duration and disease status over time. (C) Best percent change in the sum of target disease from baseline. (D) Percent change in the sum of target disease from baseline.

At a median disease surveillance follow-up of 11.4 months (range: 0.2–41.7), 19 patients had progressed, and 3 patients had died without radiographic evidence of progression. Median PFS was 14.8 months (95% CI: 6.0–20.7) and PFS at 12 months was 50% (95% CI: 32–65%, Figure 2A). At a median survival follow-up of 19.6 months (range: 3.6–86.0), 20 patients had died. Median OS was 19.6 months (95% CI: 12.0–40.6) and OS at 12 months was 71% (95% CI: 5283%, Figure 2B).

Figure 2:

(A) Progression-free survival and (B) overall survival Kaplan-Meier curves and 95% pointwise confidence intervals.

Correlatives

Among all 35 patients treated in this trial, increasing doses of SLM resulted in higher plasma concentrations over time, with 4000 μg conferring the highest plasma concentrations (Figure 3A). While numerically higher, a statistically significant difference between 4000 μg and 2500 μg (p=0.10) and 3000 μg (p=0.17) was not evidenced, which may be attributable to the limited number of patients treated at the lower dose levels.

Figure 3:

(A) Plasma selenium concentrations by dose and time. (B) The association between plasma selenium and TGFβ1 concentrations. (C) The association between tumor and plasma selenium concentrations. (D) The association between tumor and plasma axitinib concentrations.

Plasma collected on day 1 prior to SLM, and on day 14 and at cycle 3 from 10 patients treated at the 4000 μg dose level were analyzed to quantify TGF-β1 and SLM levels. Plasma SLM concentrations significantly increased (p<0.01) and TGF-β1 significantly decreased (p<0.01) over time. In addition, there was a moderate association between plasma SLM concentrations and TGF-β1 (Spearman ρ=−0.67, Figure 3B).

Plasma and tumor samples collected on day 1 prior to SLM, and on day 14 and at cycle 3 from 7 patients treated at the 4000 μg dose level were analyzed to quantify the plasma and tissue concentrations of SLM and axitinib. Selenium concentrations significantly increased in the plasma (p<0.01) and tumor (p<0.01) over time. In addition, there was a strong association between plasma and tumor selenium concentrations (Spearman ρ=0.80, Figure 3C). However, axitinib concentrations did not significantly increase in the plasma (p=0.26) or tumor (p=0.26) between day 14 and cycle 3. There was a moderate association between plasma and tumor axitinib concentrations (Spearman ρ=0.54, Figure 3D).

Fifteen patients (95 lesions) were imaged with DECT prior to study treatment initiation, and on day 14 and at the beginning of cycle 3. The average iodine density across lesions within a patient at each time point was calculated and compared over time. Iodine density changed over time with a significant decline from day 14 to cycle 3 being noted (p<0.01, Figure 4A). Statistically significant changes in iodine density over time were not evidenced for normal lung (p=0.67) and normal liver (p=0.08) tissues (Figure 4A). Pre-treatment iodine density was significantly lower for non-responders compared to responders (p=0.05, Figure 4B). The change in iodine density over time significantly differed by overall response (p=0.02) with responders experiencing a significantly larger decline in iodine density between day 14 and cycle 3 compared to non-responders (p=0.01, Figure 4B).

Figure 4:

(A) Iodine densities in tumors and normal lung and liver tissues over time. (B) Iodine densities over time by overall response.

Discussion

The treatment landscape of metastatic ccRCC has changed dramatically with the approval of therapies targeting VEGF, mammalian target of rapamycin (mTOR) pathways, as well as immune checkpoint inhibitors. In the randomized phase III AXIS trial, axitinib as a second line therapy has shown a significantly better PFS as compared to sorafenib in advanced ccRCC (7), however without improvement in OS (6).

Selenium is an essential trace mineral. The recommended dietary allowance for adults is 55 μg per day (17). Dependent on the type and dose, selenium acts as an antioxidant or a prooxidant (10). Clinical results from prevention trials with a SLM dose of 200 μg per day are mixed (18), and one large trial evaluating selenium in reducing the risk of prostate cancer showed no benefit (19). Unlike cancer prevention trials, the SLM dose used in our trial is up to 20-fold higher. The rationale to evaluate the therapeutic potential of SLM stems from the preclinical evidence suggesting that SLM facilitates the degradation of both HIFs; interact directly and inhibits TGFβ1 (8) and promotes tumor vascular stabilization resulting in enhanced delivery of drugs to tumor cells (12–14).

We have demonstrated in several preclinical xenograft models that therapeutic synergy with multiple chemotherapeutic drugs and targeted therapies are highly dependent on selenium type, dose, and schedule (20). Collectively, the preclinical mechanistic and therapeutic data provided the rationale for the development of this phase I clinical trial of SLM in sequential combination with axitinib in patients with previously treated, advanced or metastatic ccRCC.

SLM at 4000 μg was determined to be safe when combined with the standard of care axitinib; the adverse events observed in this clinical trial are consistent with those previously reported for axitinib monotherapy. Grade 3 hypertension was observed in 12 patients (34%) with the combination and was mitigated by axitinib dose reduction. Preliminary efficacy estimates in this trial suggest the addition of SLM may increase efficacy relative to axitinib alone. Although crosstrial comparisons are complicated by differences in study design and patient population, the ORR (55.6%) and median PFS (14.8 months) compared favorably to that evidenced in the AXIS trial (ORR: 19.4%, median: PFS 8.3 months) (7). However, median OS was comparable (19.6 vs 20.1 months) (6) which might be impacted by the other treatment options patient received prior and after SLM- axitinib therapy. Notably, 2 patients on trial had a radiographic complete response, and 1 patient had a pathologic complete response in this trial. The first patient had previously progressed on pazopanib with retroperitoneal metastasis adjacent to the right adrenal gland which resolved completely with study treatment. The patient is still disease-free 6 years later. The second patient had prior treatment with a combination of ipilimumab and nivolumab with lung, retroperitoneal, hilar, and subcarinal lymph node metastases. The patient had a complete response for 26 months before relapsing. The third patient had disease progression on prior treatment with a combination of ipilimumab and nivolumab. The patient had a partial response to study treatment per RECIST 1.1 criteria with multiple soft tissue pelvic masses which became seemingly necrotic within 12 months of starting treatment. The patient subsequently underwent surgical resection revealing pathologic CR (pCR) which is maintained 3 years later.

Data generated in this trial demonstrated that blood selenium concentrations, previously determined to be synergistic with axitinib in a xenograft model, can be achieved clinically without additional toxicity. As plasma selenium concentrations increased, TGF-β1 concentrations decreased suggesting it is a potential therapeutic target. Plasma and tumor concentrations of selenium were time-dependent and strongly associated demonstrating SLM is reaching and accumulating in the tumor. DECT showed an initial increase of iodine density in tumor tissues during the SLM run-in followed by a significant decline after initiation of combination treatment. This trend was not evident for normal lung and liver tissues. Preliminary results in these patients are consistent with the data generated in xenograft models suggesting that a therapeutically and molecularly effective dose of SLM may contribute to stabilization of tumor associated vasculature via increased iodine density, while having minimal impact on normal tissues, that may result in selective delivery of drug to tumor tissues (13,14).

While the results are limited by the small number of patients, the specific trends highlighted warrant further confirmatory investigation in a larger randomized phase 2 clinical trial in refractory ccRCC.

Translational Relevance.

Clinical benefit was observed when seleno-L-methionine (SLM) is combined with axitinib. SLM is being developed clinically not as a cytotoxic drug, but as selective modulator of targets associated with drug resistance and increased tumor angiogenesis. The SLM dose used in combination with axitinib is 20-fold higher than those used in prevention trials. Inhibition of TGFβ1 by increasing SLM concentrations offers the potential for the development of new and novel therapy not only for patients with advanced clear cell renal cell carcinoma (ccRCC) but also to other cancers with similar SLM target expression. The results generated in this clinical trial demonstrated that the plasma selenium concentrations derived from the SLM dose, determined to be synergistic with anticancer therapies in xenografts, was achieved in ccRCC patients treated with the combination of SLM and axitinib without added toxicity.