Biological evaluation of 3-[(18)F]fluoro-α-methyl-D-tyrosine (D-[(18)F]FAMT) as a novel amino acid tracer for positron emission tomography

Assessing the effectiveness of fluorinated and α-methylated 3-boronophenylalanine for improved tumor-specific boron delivery in boron neutron capture therapy

A [10B]boron agent and a nuclear imaging probe for pharmacokinetic estimation form the fundamental pair in successful boron neutron capture therapy (BNCT). However, 4-[10B]borono-l-phenylalanine (BPA), used in clinical BNCT, has undesirable water solubility and tumor selectivity. Therefore, we synthesized fluorinated and α-methylated 3-borono-l-phenylalanine (3BPA) derivatives to realize improved water solubility, tumor targetability, and biodistribution. All 3BPA derivatives exhibited over 10 times higher water solubility than BPA. Treatment with α-methylated 3BPA derivatives resulted in decreased cell uptake via l-type amino acid transporter (LAT) 2 while maintaining LAT1 recognition, thereby significantly improving LAT1/LAT2 selectivity. Biodistribution studies showed that fluorinated α-methyl 3BPA derivatives exhibited reduced boron accumulation in nontarget tissues, including muscle, skin, and plasma. Consequently, these derivatives demonstrated significantly improved tumor-to-normal tissue ratios compared to 3BPA and BPA. Overall, fluorinated α-methyl 3BPA derivatives with the corresponding radiofluorinated compounds hold potential as promising agents for future BNCT/PET theranostics.

Enhancing the Therapeutic Effect of 2-211At-astato-α-methyl-L-phenylalanine with Probenecid Loading

Simple Summary

To enhance the therapeutic effect of 2-²¹¹At-astato-α-methyl-L-phenylalanine (2-²¹¹At-AAMP), a radiopharmaceutical for targeted alpha therapy, we evaluated the effect of probenecid loading on its biodistribution and therapeutic effect in mice. Probenecid preloading significantly delayed the clearance of 2-²¹¹At-AAMP from the blood, increasing its accumulation in tumors. Consequently, the therapeutic effect of 2-²¹¹At-AAMP markedly improved. These results indicate that 2-²¹¹At-AAMP with probenecid loading is useful for the treatment of various types of cancers.

Abstract

L-type amino acid transporter 1 (LAT1) might be a useful target for tumor therapy since it is highly expressed in various types of cancers. We previously developed an astatine-211 (²¹¹At)-labeled amino acid derivative, 2-²¹¹At-astato-α-methyl-L-phenylalanine (2-²¹¹At-AAMP), and demonstrated its therapeutic potential for LAT1-positive cancers. However, the therapeutic effect of 2-²¹¹At-AAMP was insufficient, probably due to its low tumor retention. The preloading of probenecid, an organic anion transporter inhibitor, can delay the clearance of some amino acid tracers from the blood and consequently increase their accumulation in tumors. In this study, we evaluated the effect of probenecid preloading on the biodistribution and therapeutic effect of 2-²¹¹At-AAMP in mice. In biodistribution studies, the blood radioactivity of 2-²¹¹At-AAMP significantly increased with probenecid preloading. Consequently, the accumulation of 2-²¹¹At-AAMP in tumors was significantly higher with probenecid than without probenecid loading. In a therapeutic study, tumor growth was suppressed by 2-²¹¹At-AAMP with probenecid, and the tumor volume was significantly lower in the treatment group than in the untreated control group from day 2 to day 30 (end of the follow-up period) after treatment. These results indicate that probenecid loading could improve the therapeutic effect of 2-²¹¹At-AAMP by increasing its accumulation in tumors.

18F-Trifluoromethylated D-Cysteine as a Promising New PET Tracer for Glioma Imaging: Comparative Analysis With MRI and Histopathology in Orthotopic C6 Models

Comparing MRI and histopathology, this study aims to comprehensively explore the potential application of ¹⁸F-trifluoromethylated D-cysteine (S-[¹⁸F]CF₃-D-CYS) in evaluating glioma by using orthotopic C6 glioma models. Sprague-Dawley (SD) rats (n = 9) were implanted with C6 glioma cells. Tumor growth was monitored every week by multiparameter MRI [including dynamic contrast-enhanced MRI (DCE-MRI)], [¹⁸F]FDG, S-[¹⁸F]CF₃-D-CYS, and [¹⁸F]FDOPA PET imaging. Repeated scans of the same rat with the two or three [¹⁸F]-labeled radiotracers were investigated. Initial regions of interest were manually delineated on T₂WI and set on the same level of PET images, and tumor-to-normal brain uptake ratios (TNRs) were calculated to semiquantitatively assess the tracer accumulation in the tumor. The tumor volume in PET and histopathology was calculated. HE and Ki67 immunohistochemical staining were further performed. The correlations between the uptake of S-[¹⁸F]CF₃-D-CYS and Ki67 were analyzed. Dynamic S-[¹⁸F]CF₃-D-CYS PET imaging showed tumor uptake rapidly reached a peak, maintained plateau during 10-30 min after injection, then decreased slowly. Compared with [¹⁸F]FDG and [¹⁸F]FDOPA PET imaging, S-[¹⁸F]CF₃-D-CYS PET demonstrated the highest TNRs (P < 0.05). There were no significant differences in the tumor volume measured on S-[¹⁸F]CF₃-D-CYS PET or HE specimen. Furthermore, our results showed that the uptake of S-[¹⁸F]CF₃-D-CYS was significantly positively correlated with tumor Ki67, and the poor accumulated S-[¹⁸F]CF₃-D-CYS was consistent with tumor hemorrhage. There was no significant correlation between the S-[¹⁸F]CF₃-D-CYS uptakes and the K^trans values derived from DCE-MRI. In comparison with MRI and histopathology, S-[¹⁸F]CF₃-D-CYS PET performs well in the diagnosis and evaluation of glioma. S-[¹⁸F]CF₃-D-CYS PET may serve as a valuable tool in the clinical management of gliomas.

Preclinical evaluation of new α-radionuclide therapy targeting LAT1: 2-[211At]astato-α-methyl-L-phenylalanine in tumor-bearing model

INTRODUCTION

Targeted α-radionuclide therapy has attracted attention as a promising therapy for refractory cancers. However, the application is limited to certain types of cancer. Since L-type amino acid transporter 1 (LAT1) is highly expressed in various human cancers, we prepared an LAT1-selective α-radionuclide-labeled amino acid analog, 2-[211At]astato-α-methyl-L-phenylalanine (2-[211At]AAMP), and evaluated its potential as a therapeutic agent.

METHODS

2-[211At]AAMP was prepared from the stannyl precursor. Stability of 2-[211At]AAMP was evaluated both in vitro and in vivo. In vitro studies using an LAT1-expressing human ovarian cancer cell line, SKOV3, were performed to evaluate cellular uptake and cytotoxicity of 2-[211At]AAMP. Biodistribution and therapeutic studies in SKOV3-bearing mice were performed after intravenous injection of 2-[211At]AAMP.

RESULTS

2-[211At]AAMP was stable in murine plasma in vitro and excreted intact into urine. Cellular uptake of 2-[211At]AAMP was inhibited by treatment with an LAT1-selective inhibitor. After 24 h incubation, 2-[211At]AAMP suppressed clonogenic growth at 10 kBq/ml, and induced cell death and DNA double-strand breaks at 25 kBq/ml. When injected into mice, 2-[211At]AAMP exhibited peak accumulation in the tumor at 30 min postinjection, and radioactivity levels in the tumor were retained up to 60 min. The majority of the radioactivity was rapidly eliminated from the body into urine in an intact form immediately after injection. 2-[211At]AAMP significantly improved the survival of mice (P < 0.05) without serious side effects.

CONCLUSION

2-[211At]AAMP showed α-radiation-dependent cellular growth inhibition after it was taken up via LAT1. In addition, 2-[211At]AAMP had a beneficial effect on survival in vivo. These findings suggest that 2-[211At]AAMP would be useful for the treatment of LAT1-positive cancer.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

This is the first report of an LAT1-targeting radiopharmaceutical for α-radionuclide therapy; this agent would be applicable for the treatment of various types of cancer.

Phase II trial of SM-88, a cancer metabolism based therapy, in non-metastatic biochemical recurrent prostate cancer

Background Androgen deprivation therapy (ADT) is a standard treatment for high-risk biochemically-recurrent, non-metastatic prostate cancer (BRPC) but is not curative and associated with toxicity. Racemetyrosine (SM-88) is an amino-acid analogue used with methoxsalen, phenytoin, and sirolimus (MPS) to enhance SM-88 activity. Method A phase 1b/2, open-label trial in BRPC and rising PSA. Patients were given daily SM-88 (230 mg BID), methoxsalen (10 mg), phenytoin (50 mg), and sirolimus (0.5 mg)). Outcome measures included changes in PSA, circulating tumor cells (CTCs) and imaging. Results 34 subjects were screened, 23 treated and 21 remained on study for ≥12 weeks. The median PSA was 6.4 ng/ml (range 1.7-80.1); doubling-time 6.2 months (range 1.4-36.6) and baseline testosterone 319.1 ng/ml (range 2.5-913.7). Median duration of therapy was 6.5 months (2.6-14.0). CTCs (median 48.5 cells/4 ml (range 15-268) at baseline) decreased a median of 65.3% in 18 of 19 patients. For patients who achieved an absolute CTC nadir count of <10 cells/4 ml (n = 10), disease control was 100% i.e. no metastases or PSA progression, while on trial (p = 0.005). PSA fell by ≥50% in 4.3% (1 subject). No patients developed metastatic disease while on treatment (metastases free survival =100%). There were no treatment-related adverse events (AEs) and quality of life was unchanged from baseline on the EORTC QLQ-C30 and QLQ-PR25. Testosterone levels rose slightly on SM-88 and were unrelated to efficacy or toxicity. Conclusions Use of SM-88 was associated with disease control while maintaining QOL. SM-88 may delay the need for ADT and the associated hormonal side effects. Larger trials are planned.Trial registration number, date of registration - NCT02796898, June 13, 2016.

Diagnosis of bladder cancer using 18F-labeled α-methyl-phenylalanine tracers in a mouse model

OBJECTIVE

Although 2-¹⁸F-fluoro-2-deoxy-glucose (¹⁸F-FDG) has established roles in the diagnosis of a variety of cancers, it has limited value in the detection of primary/recurrent lesions in the bladder, mainly because of interference by the pooled radioactivity in the urine. Our previous study revealed promising properties of L- and D-2-¹⁸F-α-methyl-phenylalanine (2-¹⁸F-FAMP) as radiotracers; i.e., their rapid blood clearance and low renal accumulation. In the present study we evaluated the utility of L- and D-2-¹⁸F-FAMP for imaging bladder cancer in a mouse model.

METHODS

We used the human bladder cancer cell line HT1376 to prepare a bladder cancer xenograft model in mice bearing both orthotopic and subcutaneous tumors. Biodistribution and PET imaging studies were performed at 1 and 3 h after injection of L-2-¹⁸F-FAMP or D-2-¹⁸F-FAMP. ¹⁸F-FDG was used as a control.

RESULTS

At 1 h after injection, greater accumulations of both L-2-¹⁸F-FAMP and D-2-¹⁸F-FAMP were observed in the orthotopic tumors compared to ¹⁸F-FDG. The orthotopic tumor-to-muscle ratio of D-2-¹⁸F-FAMP was significantly higher than that of ¹⁸F-FDG (p < 0.01), because of the rapid blood clearance of D-2-¹⁸F-FAMP. L-2-¹⁸F-FAMP showed the highest subcutaneous tumor-to-muscle ratio (p < 0.01) due to its high subcutaneous tumor uptake. Compared to L-2-¹⁸F-FAMP, D-2-¹⁸F-FAMP exhibited faster clearance and lower kidney accumulation. In the PET imaging studies, L- and D-2-¹⁸F-FAMP both clearly visualized the orthotopic bladder tumors at 1 h after injection.

CONCLUSION

Our study showed that L-2-¹⁸F-FAMP and D-2-¹⁸F-FAMP have the potential to detect bladder cancer.

New fluoroethyl phenylalanine analogues as potential LAT1-targeting PET tracers for glioblastoma

The use of O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET) as a positron emission tomography (PET) tracer for brain tumor imaging might have some limitations because of the relatively low affinity for the L-type amino acid transporter 1 (LAT1). To assess the stereospecificity and evaluate the influence of aromatic ring modification of phenylalanine LAT1 targeting tracers, six different fluoroalkylated phenylalanine analogues were synthesized. After in vitro Ki determination, the most promising compound, 2-[18F]-2-fluoroethyl-L-phenylalanine (2-[18F]FELP), was selected for further evaluation and in vitro comparison with [18F]FET. Subsequently, 2-[18F]FELP was assessed in vivo and compared with [18F]FET and [18F]FDG in a F98 glioblastoma rat model. 2-[18F]FELP showed improved in vitro characteristics over [18F]FET, especially when the affinity and specificity for system L is concerned. Based on our results, 2-[18F]FELP is a promising new PET tracer for brain tumor imaging.

Reevaluating the Substrate Specificity of the L-Type Amino Acid Transporter (LAT1)

The L-type amino acid transporter 1 (LAT1, SLC7A5) transports essential amino acids across the blood-brain barrier (BBB) and into cancer cells. To utilize LAT1 for drug delivery, potent amino acid promoieties are desired, as prodrugs must compete with millimolar concentrations of endogenous amino acids. To better understand ligand-transporter interactions that could improve potency, we developed structural LAT1 models to guide the design of substituted analogues of phenylalanine and histidine. Furthermore, we evaluated the structure-activity relationship (SAR) for both enantiomers of naturally occurring LAT1 substrates. Analogues were tested in cis-inhibition and trans-stimulation cell assays to determine potency and uptake rate. Surprisingly, LAT1 can transport amino acid-like substrates with wide-ranging polarities including those containing ionizable substituents. Additionally, the rate of LAT1 transport was generally nonstereoselective even though enantiomers likely exhibit different binding modes. Our findings have broad implications to the development of new treatments for brain disorders and cancer.

Prostate Cancer Cells in Different Androgen Receptor Status Employ Different Leucine Transporters

BACKGROUND

Leucine stimulates cancer cell proliferation through the mTOR pathway, therefore, inhibiting leucine transporters may be a novel therapeutic target for cancer. L-type amino acid transporter (LAT) 1, a Na⁺ -independent amino acid transporter, is highly expressed in many tumor cells. However, leucine transporter(s) in different stages of prostate cancer, particularly in the stages of castration resistance with androgen receptor (AR) expression, is unclear.

METHODS

LNCaP and DU145 and PC-3 cell lines were used as a model of androgen dependent, and metastatic prostate cancer. A new "LN-cr" cell line was established after culturing LNCaP cells for 6 months under androgen-free conditions, which is considered a model of castration resistant prostate cancer (CRPC) with androgen AR expression. The expression of leucine transporters was investigated with quantitative PCR and immunofluorescence. Uptake of ¹⁴ C Leucine was examined in the presence or absence of BCH (a pan-LAT inhibitor), JPH203 (an LAT1-specific inhibitor), or Na⁺ . Cell growth was assessed with MTT assay. siRNA studies were performed to evaluate the indispensability of y⁺ LAT2 on leucine uptake and cell viability in LN-cr.

RESULTS

Cell viability showed a 90% decrease in the absence of leucine in all four cell lines. LNCaP cells principally expressed LAT3, and their leucine uptake was more than 90% Na⁺ -independent. BCH, but not JPH203, inhibited leucine uptake, and cell proliferation (IC_50BCH :15 mM). DU145 and PC-3 cells predominantly expressed LAT1. Leucine uptake and cell growth were suppressed by BCH or JPH203 in a dose-dependent manner (IC_50BCH : ∼20 mM, IC_50JPH203 : ∼5 µM). In LN-cr cells, Na⁺ -dependent uptake of leucine was 3.8 pmol/mgprotein/min, while, Na⁺ -independent uptake was only 0.52 (P < 0.05). Leucine uptake of LN-cr was largely (∼85%) Na⁺ -dependent. y⁺ LAT2 expression was confirmed in LN-cr. Knockdown of y⁺ LAT2 lead to significant leucine uptake inhibition (40%) and cell growth inhibition (20%).

CONCLUSIONS

New CRPC cell line with increased expression of y⁺ LAT2 as a leucine transporter was established in vitro. Anti-leucine transporter therapy could be an important option against prostate cancer. Prostate 77:222-233, 2017. © 2016 Wiley Periodicals, Inc.

18F-FBPA as a tumor-specific probe of L-type amino acid transporter 1 (LAT1): a comparison study with 18F-FDG and 11C-Methionine PET

PURPOSE

The purpose of this study was to evaluate the usefulness of L-4-borono-2-¹⁸F-fluoro-phenylalanine (¹⁸F-FBPA) as a tumor-specific probe, in comparison to ¹⁸F-FDG and ¹¹C-methionine (Met), focusing on its transport selectivity by L-type amino acid transporter 1 (LAT1), which is highly upregulated in cancers.

METHODS

Cellular analyses of FBPA were performed to evaluate the transportablity and K_m value. PET studies were performed in rat xenograft models of C6 glioma (n = 12) and in rat models of turpentine oil-induced subcutaneous inflammation (n = 9). The kinetic parameters and uptake values on static PET images were compared using the one-tissue compartment model (K₁, k₂) and maximum standardized uptake value (SUVmax).

RESULTS

The cellular analyses showed that FBPA had a lower affinity to a normal cell-type transporter LAT2 and induced less efflux through LAT2 among FBPA, Met, and BPA, while the efflux through LAT1 induced by FBPA was similar among the three compounds. The K_m value of ¹⁸F-FBPA for LAT1 (196.8 ± 11.4 μM) was dramatically lower than that for LAT2 (2813.8 ± 574.5 μM), suggesting the higher selectivity of ¹⁸F-FBPA for LAT1. K₁ and k₂ values were significantly smaller in ¹⁸F-FBPA PET (K₁ = 0.04 ± 0.01 ml/ccm/min and k₂ = 0.07 ± 0.01 /min) as compared to ¹¹C-Met PET (0.22 ± 0.09 and 0.52 ± 0.10, respectively) in inflammatory lesions. Static PET analysis based on the SUVmax showed significantly higher accumulation of ¹⁸F-FDG in the tumor and inflammatory lesions (7.2 ± 2.1 and 4.6 ± 0.63, respectively) as compared to both ¹⁸F-FBPA (3.2 ± 0.40 and 1.9 ± 0.19) and ¹¹C-Met (3.4 ± 0.43 and 1.6 ± 0.11). No significant difference was observed between ¹⁸F-FBPA and ¹¹C-Met in the static PET images.

CONCLUSION

This study shows the utility of ¹⁸F-FBPA as a tumor-specific probe of LAT1 with low accumulation in the inflammatory lesions.

Characterization of 5-(2- 18F-fluoroethoxy)-L-tryptophan for PET imaging of the pancreas

Purpose: In diabetes, pancreatic beta cell mass declines significantly prior to onset of fasting hyperglycemia. This decline may be due to endoplasmic reticulum (ER) stress, and the system L amino acid transporter LAT1 may be a biomarker of this process. In this study, we used 5-(2- ¹⁸F-fluoroethoxy)-L-tryptophan ( ¹⁸F-L-FEHTP) to target LAT1 as a potential biomarker of beta cell function in diabetes.

Procedures: Uptake of ¹⁸F-L-FEHTP was determined in wild-type C57BL/6 mice by ex vivo biodistribution. Both dynamic and static positron emission tomography (PET) images were acquired in wild-type and Akita mice, a model of ER stress-induced diabetes, as well as in mice treated with streptozotocin (STZ). LAT1 expression in both groups of mice was evaluated by immunofluorescence microscopy.

Results: Uptake of ¹⁸F-L-FEHTP was highest in the pancreas, and static PET images showed highly specific pancreatic signal. Time-activity curves showed significantly reduced ¹⁸F-L-FEHTP uptake in Akita mice, and LAT1 expression was also reduced. However, mice treated with STZ, in which beta cell mass was reduced by 62%, showed no differences in ¹⁸F-L-FEHTP uptake in the pancreas, and there was no significant correlation of ¹⁸F-L-FEHTP uptake with beta cell mass.

Conclusions: ¹⁸F-L-FEHTP is highly specific for the pancreas with little background uptake in kidney or liver. We were able to detect changes in LAT1 in a mouse model of diabetes, but these changes did not correlate with beta cell function or mass. Therefore, ¹⁸F-L-FEHTP PET is not a suitable method for the noninvasive imaging of changes in beta cell function during the progression of diabetes.

Characterization of 5-(2- 18F-fluoroethoxy)-L-tryptophan for PET imaging of the pancreas

Purpose: In diabetes, pancreatic beta cell mass declines significantly prior to onset of fasting hyperglycemia. This decline may be due to endoplasmic reticulum (ER) stress, and the system L amino acid transporter LAT1 may be a biomarker of this process. In this study, we used 5-(2- 18F-fluoroethoxy)-L-tryptophan ( 18F-L-FEHTP) to target LAT1 as a potential biomarker of beta cell function in diabetes. Procedures: Uptake of 18F-L-FEHTP was determined in wild-type C57BL/6 mice by ex vivo biodistribution. Both dynamic and static positron emission tomography (PET) images were acquired in wild-type and Akita mice, a model of ER stress-induced diabetes, as well as in mice treated with streptozotocin (STZ). LAT1 expression in both groups of mice was evaluated by immunofluorescence microscopy. Results: Uptake of 18F-L-FEHTP was highest in the pancreas, and static PET images showed highly specific pancreatic signal. Time-activity curves showed significantly reduced 18F-L-FEHTP uptake in Akita mice, and LAT1 expression was also reduced. However, mice treated with STZ, in which beta cell mass was reduced by 62%, showed no differences in 18F-L-FEHTP uptake in the pancreas, and there was no significant correlation of 18F-L-FEHTP uptake with beta cell mass. Conclusions:18F-L-FEHTP is highly specific for the pancreas with little background uptake in kidney or liver. We were able to detect changes in LAT1 in a mouse model of diabetes, but these changes did not correlate with beta cell function or mass. Therefore, 18F-L-FEHTP PET is not a suitable method for the noninvasive imaging of changes in beta cell function during the progression of diabetes.

Using Metabolomic Profiles as Biomarkers for Insulin Resistance in Childhood Obesity: A Systematic Review

A growing body of evidence has shown the intimate relationship between metabolomic profiles and insulin resistance (IR) in obese adults, while little is known about childhood obesity. In this review, we searched available papers addressing metabolomic profiles and IR in obese children from inception to February 2016 on MEDLINE, Web of Science, the Cochrane Library, ClinicalTrials.gov, and EMASE. HOMA-IR was applied as surrogate markers of IR and related metabolic disorders at both baseline and follow-up. To minimize selection bias, two investigators independently completed this work. After critical selection, 10 studies (including 2,673 participants) were eligible and evaluated by using QUADOMICS for quality assessment. Six of the 10 studies were classified as "high quality." Then we generated all the metabolites identified in each study and found amino acid metabolism and lipid metabolism were the main affected metabolic pathways in obese children. Among identified metabolites, branched-chain amino acids (BCAAs), aromatic amino acids (AAAs), and acylcarnitines were reported to be associated with IR as biomarkers most frequently. Additionally, BCAAs and tyrosine seemed to be relevant to future metabolic risk in the long-term follow-up cohorts, emphasizing the importance of early diagnosis and prevention strategy. Because of limited scale and design heterogeneity of existing studies, future studies might focus on validating above findings in more large-scale and longitudinal studies with elaborate design.

Metformin enhances anti-tumor effect of L-type amino acid transporter 1 (LAT1) inhibitor

BACKGROUND

In many cancer cells, L-type amino acid transporter 1 (LAT1) transports neutral amino acids with bulky side chain, which activate mammalian target of rapamycin (mTOR) to cause cell proliferation. An anti-diabetic drug, metformin, has been shown to activate AMP-activated protein kinase (AMPK), which leads to inhibition of mTOR. LAT1 inhibition in combination with metformin could result in more prominent suppression of mTOR activity.

PURPOSE

Anti-proliferative effect of a newly developed LAT1 specific inhibitor JPH203 in combination with metformin is evaluated in 2 head and neck cancer cell lines, Ca9-22 and HEp-2 cells and in nude mice inoculated with Ca9-22 cells.

RESULTS AND DISCUSSION

By MTT assay, 0.5 mM metformin inhibited proliferation of Ca9-22 cells to 70% of control. In the presence of 100 μM JPH203, proliferation of Ca9-22 cells was inhibited to 60% of control. By combining these 2 drugs, proliferation of Ca9-22 was significantly inhibited to 40% of control. However, this regimen was not very effective against HEp-2 cells. This combination also suppressed in vivo growth of Ca9-22 cells in a xenotransplant model. A combination of anti-LAT1 drug with metformin may be an effective anti-proliferative therapy for certain subsets of cancers.