The role of radioactive somatostatin and its analogues in the control of tumor growth

68 Ga-DOTATATE PET/CT Versus 18 F-FDG PET/CT in TENIS Syndrome: A Head-to-Head Comparison With Elevated and Suppressed TSH Levels in Papillary Thyroid Carcinoma-A Pilot Study

BACKGROUND

TENIS syndrome is characterized by reduced expression of sodium-iodine symporter, rising serum thyroglobulin (Tg) levels, and negative whole-body 131 I scans. In such patients, somatostatin receptor imaging with 68 Ga-DOTATATE PET/CT (somatostatin receptor [SSR] PET/CT) and 18 F-FDG PET/CT (FDG PET/CT) can identify metastases and were compared under 2 conditions: elevated (eTSH) and suppressed (sTSH) TSH serum levels. Potential candidates for peptide receptor radionuclide therapy (PRRNT) were identified in 15 patients prospectively enrolled. All patients underwent 4 examinations. Images were blindly evaluated for differences in SUV max values and lesion detectability. Reference standard consisted of neck ultrasound, CT, MRI, PET/CT, biopsy, and follow-up. Three patients were received PRRNT.

RESULTS

sTSH SSR PET/CT detected a greater number of cervical ( P = 0.0253 and P = 0.0176) and distant LNs ( P = 0.0253 and P = 0.0391) when compared with sTSH FDG PET/CT, respectively, in a per-patient and on a per-lesion based analysis. Likewise, eTSH SSR PET/CT detected a greater number of patients with local recurrences ( P = 0.0455) and distant LN metastases ( P = 0.0143). Per-lesion analysis revealed greater number of cervical and distant LNs ( P = 0.0337 and P = 0.0039, respectively) when compared with eTSH FDG PET/CT. There was no difference in detection of distant metastases by both tracers for lung and bone metastases (κ = 1). Both skeletal and pulmonary lesions were also detected by conventional CT part of FDG or DOTATATE PET/CT scans. TSH stimulation had no additional value in a per-patient analysis for both FDG and DOTATATE PET scans (κ varying from 0.6087 to 1). However, TSH stimulation led to more lesion identifications in DOTATATE PET/CT; most of those metastases were not confirmed by the reference standard leading to a decrease in specificity (84% vs 74%). One of 3 patients submitted to 3 cycles of PRRNT presented with a visual partial response, a 20% reduction in quantitative analyses, and stable disease regarding Tg and TgAb levels.

CONCLUSIONS

Patients with TENIS syndrome can be imaged with SSR PET/CT as well as FDG PET/CT with high overall accuracy regardless of TSH levels (86% to 92% and 92% to 85%, respectively, with eTSH and sTSH). SSR PET/CT detected a greater number of locoregional and distant LN metastases than FDG PET/CT with both sTSH and eTSH. One of 3 patients submitted to PRRNT presented a partial response to treatment. Our findings may impact in patient restaging, management, and theranostics strategies with radiolabeled somatostatin analogs.

Promising targetable biomarkers in pancreatic neuroendocrine tumours

INTRODUCTION

In the treatment scenario of PanNETs-targeted therapies are desired but limited, as rarity and heterogeneity on PanNETs pose limitations to their development.

AREAS COVERED

We performed a literature review searching for promising druggable biomarkers and potential treatments to be implemented in the next future. We focused on treatments which have already reached clinical experimentation, although in early phases. Six targets were identified, namely Hsp90, HIFa, HDACs, CDKs, uPAR, and DDR. Even though biological rational is strong, so far reported efficacy outcomes are quite disappointing. The reason of that should be searched in the patients' heterogeneity, lack of biomarker selection, poor knowledge of interfering mechanisms as well as difficulties in patients accrual. Moreover, different ways to assess treatment efficacy should be considered, other than response rate, in light of the more indolent nature of NETs.

EXPERT OPINION

Development of targeted treatments in PanNETs is still an uncovered area, far behind other more frequent cancers. Rarity of NETs led to accrual of unselected populations, possibly jeopardizing the drug efficacy. Better patients' selection, both in terms of topography, grading and biomarkers is crucial and will help understanding which role targeted therapies can really play in these tumors.

New Perspectives on Pheochromocytoma and Paraganglioma: Toward a Molecular Classification

A molecular biology-based taxonomy has been proposed for pheochromocytoma and paraganglioma (PPGL). Data from the Cancer Genome Atlas revealed clinically relevant prognostic and predictive biomarkers and stratified PPGLs into three main clusters. Each subgroup has a distinct molecular-biochemical-imaging signature. Concurrently, new methods for biochemical analysis, functional imaging, and medical therapies have also become available. The research community now strives to match the cluster biomarkers with the best intervention. The concept of precision medicine has been long awaited and holds great promise for improved care. Here, we review the current and future PPGL classifications, with a focus on hereditary syndromes. We discuss the current strengths and shortcomings of precision medicine and suggest a condensed manual for diagnosis and treatment of both adult and pediatric patients with PPGL. Finally, we consider the future direction of this field, with a particular focus on how advanced molecular characterization of PPGL can improve a patient's outcome, including cures and, ultimately, disease prevention.

Localized Irradiation of Cell Membrane by Auger Electrons Is Cytotoxic Through Oxidative Stress-Mediated Nontargeted Effects

AIMS

We investigated whether radiation-induced nontargeted effects are involved in the cytotoxic effects of anticell surface monoclonal antibodies labeled with Auger electron emitters, such as iodine 125 (monoclonal antibodies labeled with (125)I [(125)I-mAbs]).

RESULTS

We showed that the cytotoxicity of (125)I-mAbs targeting the cell membrane of p53(+/+) HCT116 colon cancer cells is mainly due to nontargeted effects. Targeted and nontargeted cytotoxicities were inhibited in vitro following lipid raft disruption with Methyl-β-cyclodextrin (MBCD) or filipin or use of radical oxygen species scavengers. (125)I-mAb efficacy was associated with acid sphingomyelinase activation and modulated through activation of the AKT, extracellular signal-related kinase ½ (ERK1/2), p38 kinase, c-Jun N-terminal kinase (JNK) signaling pathways, and also of phospholipase C-γ (PLC-γ), proline-rich tyrosine kinase 2 (PYK-2), and paxillin, involved in Ca(2+) fluxes. Moreover, the nontargeted response induced by directing 5-[(125)I]iodo-2'-deoxyuridine to the nucleus was comparable to that of (125)I-mAb against cell surface receptors. In vivo, we found that the statistical significance of tumor growth delay induced by (125)I-mAb was removed after MBCD treatment and observed oxidative DNA damage beyond the expected Auger electron range. These results suggest the involvement of nontargeted effects in vivo also.

INNOVATION

Low-energy Auger electrons, such as those emitted by (125)I, have a short tissue range and are usually targeted to the nucleus to maximize their cytotoxicity. In this study, we show that targeting the cancer cell surface with (125)I-mAbs produces a lipid raft-mediated nontargeted response that compensates for the inferior efficacy of non-nuclear targeting.

CONCLUSION

Our findings describe the mechanisms involved in the efficacy of (125)I-mAbs targeting the cancer cell surface. Antioxid. Redox Signal. 25, 467-484.

Systemic treatment of gastroenteropancreatic neuroendocrine tumors (GEP-NETS): current approaches and future options

OBJECTIVE

To describe recent advances in the treatment of gastroenteropancreatic neuroendocrine tumors (GEP-NETs).

METHODS

A review of the published English language literature on GEP-NET therapy with a focus on practice-changing clinical trials.

RESULTS

Somatostatin analog (SSA) treatment remains a cornerstone of GEP-NET therapy, primarily for patients with hormonally functional tumors and midgut carcinoids. The biologic agents everolimus and sunitinib have similar tumor-stabilizing effects in pancreatic NETs and are both approved to treat progressive low-intermediate-grade tumors. Their role in nonpancreatic NETs remains controversial. Cytotoxic chemotherapy is effective against pancreatic NETs, but modern prospective data is lacking. Radiolabeled SSAs will likely become more widely available once phase III randomized studies are completed.

CONCLUSIONS

New treatment options for GEP-NETs have become available and highlight the necessity of developing predictive biomarkers that will allow for appropriate and individualized therapy selection.

Therapeutic management of patients with gastroenteropancreatic neuroendocrine tumours

Patients with neuroendocrine tumours (NETs) are best managed in a specialist centre as part of a multidisciplinary team comprising gastroenterologists, oncologists, endocrinologists, gastrointestinal and hepatopancreaticobiliary surgeons, pathologists, nuclear medicine physicians and technicians, radiologists, specialist nurses, pharmacists, biochemists and dieticians. This should ideally be led by a clinician with experience and interest in NETs. Although the number of medical treatments and clinical trials has increased in the decade, there is still a lack of prospective randomised trials; thus, management is mainly based on limited often single-centre studies, although there are now formal guidelines based on consensus expert opinion. We have outlined the current optimal management of patients with NETs. We have reviewed therapeutic options including surgery, somatostatin analogues and other biotherapies and peptide receptor-targeted therapy. We have discussed the challenge in managing hepatic metastases including hepatic artery embolisation, ablation and orthotopic liver transplant. In addition, we have briefly reviewed the emerging therapies such as the mammalian target of rapamycin and angiogenic inhibitors and the newer somatostatin analogues.

Toward development of targeted nonsteroidal antiandrogen-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-gadolinium complex for prostate cancer diagnostics

Androgen receptors are present in most advanced prostate cancer specimens, having a critical role in development of this type of cancer. For correct prognosis of patient conditions and treatment monitoring, noninvasive imaging techniques have great advantages over surgical procedures. We developed synthetic methodologies for preparation of novel androgen receptor-targeting agents in an attempt to build a versatile platform for prostate cancer imaging and treatment. The structure of these compounds comprises of a lanthanoid metal ion, gadolinium-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (Gd-DOTA)-based binding fragment and, connected to it by a flexible linker, bicalutamide-derived nonsteroidal antiandrogen moiety. A representative gadolinium complex 15 was evaluated as a magnetic resonance imaging (MRI) agent in C57/bl6 male mouse bearing orthotopic TRAMP C2 prostate tumor.

Enhancement of reaction conditions for the radiolabelling of DOTA-peptides with high activities of yttrium-90

Peptide receptor radionuclide therapy (PRRT) has recently expanded due to radiolabelling of DOTA-peptides, such as the somatostatin analogues [DOTA(0), Tyr(3)]octreotate (DOTATATE). The achievement of high specific activities during procedures has been indicated as the critical factor to consent effective therapy. Several radiochemical factors may negatively impact reaction procedures such as pH, temperature and time of reaction. Our study was undertaken to explore the influence of radiochemical parameters, such as time of incubation, on reaction kinetics during the radiolabelling of DOTATATE with (90)Y.

METHODS

Forty-five radiolabelling procedures were carried out using small volumes of yttrium-90, typically 60-78 μL. At nearly constant pH and temperature two different settings of radiolabelling procedures were implemented, removing the products from the heating water bath approximately after 30 min (group E, early; n=20) and after 39 min (group L, later; n=25). Quality controls were performed by means of both high-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC).

RESULTS

Reaction kinetics for (90)Y were found to a provide suitable percentage of incorporation at pH 4.5 for both groups. Reaction temperature was not different between groups E and L. A significant difference was found between the two groups in radiochemical yield, which was 95.6% ± 0.8 for group E and 98.2% ± 1.1 for group L (p<0.0001). The specific activity of the final product was 46.9 MBq/nmol.

CONCLUSION

In order to achieve optimal specific activities, pH, temperature and time of reaction necessitate careful evaluation and setting. A statistically significant difference in labelling yield was found between a set of procedures completed at 39 min as compared to that executed at 30 min, keep the reaction pH and temperature constant.

Tumors of the Pancreatic Body and Tail

Tumors of the pancreatic body and tail are uncommon. They have a propensity to present late and often attain a large size with local invasion before they produce any clinical symptoms. The current review aims at comprehensively analysing these tumors with respect to their pathology, presentation, the investigation of these tumors, and finally the latest trends in their surgical and medical management.

Embolization therapy of bone metastases from epithelial thyroid carcinoma: effect on symptoms and serum thyroglobulin

BACKGROUND

Selective embolization therapy (SET) has been employed to treat a number of malignant tumors, but experience with its use in metastatic epithelial thyroid carcinoma (ETC) is limited. Here we report our experience with the effect of SET on symptoms and serum thyroglobulin (Tg) in patients with bone metastases from ETC.

METHODS

This was a retrospective study of 13 patients with bone metastases from ETC who underwent 65 embolizations for bone metastases in 43 sessions. In the treated patients, symptoms considered related to bone metastases were compared before and about 4-7 weeks after the embolization session. Embolization sessions were excluded for analysis if concomitant therapy had taken place within 4-7 weeks before and/or after the session. Serum Tg concentrations were employed as an index of tumor debulking by SET. We attempted to estimate the influence of SET on survival time in patients with disseminated ETC who did, and an historical control group of patients with disseminated ETC who did not receive SET.

RESULTS

After exclusion of 12 (of which 3 sessions failed) out of 43 sessions, clinical symptoms, such as pain, and neurological symptoms decreased after 17, increased after 6, and did not change after 8 procedures. In 43 sessions, 20 of which were excluded (including the 3 sessions that failed), serum Tg decreased after 14 and increased after 9. The median standardized survival time of the group that received embolization was not significantly different to that of the group that did not receive embolization.

CONCLUSIONS

Embolization therapy does not appear to improve life expectancy, but in selected patients can achieve palliation of pain, prevent neurological damage, reduce tumor burden, and give devascularization of the tumor before surgery.

Evaluation of the Therapeutic Response to In-111-DTPA Octreotide-Based Targeted Therapy in Liver Metastatic Neuroendocrine Tumors According to CT/MRI/US Findings

Thirteen (13) patients with liver neuroendocrine carcinomas metastases, positive for somatostatin receptors, confirmed by scintigraphy were infused with 4070-7030 MBq per session of In-111-octreotide after selective hepatic catheterization, exploiting the catastrophic activity of Indium Auger and Internal Conversion electron emission on cell DNA. Evaluation of the treatment was assessed by ultrasonography (US) as well as by computed tomography and/or magnetic resonance imaging scans. US appears to be the imaging procedure of choice because the examination is sensitive for evaluating lesions' edema and cystic components, provides precise measurement of tumor size, and is inexpensive. Degeneration US signs were classified in stage I (an echolucent rim in the periphery of the lesion), stage IIa (lesion with large cystic spaces), stage IIb (tiny cystic spaces) and stage III (absorption of the cystic component or stable cystic remnants).

Global gene expression in neuroendocrine tumors from patients with the MEN1 syndrome

Background

Multiple Endocrine Neoplasia type 1 (MEN1, OMIM 131100) is an autosomal dominant disorder characterized by endocrine tumors of the parathyroids, pancreatic islets and pituitary. The disease is caused by the functional loss of the tumor suppressor protein menin, coded by the MEN1 gene. The protein sequence has no significant homology to known consensus motifs. In vitro studies have shown menin binding to JunD, Pem, Smad3, NF-kappaB, nm23H1, and RPA2 proteins. However, none of these binding studies have led to a convincing theory of how loss-of-menin leads to neoplasia.

Results

Global gene expression studies on eight neuroendocrine tumors from MEN1 patients and 4 normal islet controls was performed utilizing Affymetrix U95Av2 chips. Overall hierarchical clustering placed all tumors in one group separate from the group of normal islets. Within the group of tumors, those of the same type were mostly clustered together. The clustering analysis also revealed 19 apoptosis-related genes that were under-expressed in the group of tumors. There were 193 genes that were increased/decreased by at least 2-fold in the tumors relative to the normal islets and that had a t-test significance value of p < = 0.005. Forty-five of these genes were increased and 148 were decreased in the tumors relative to the controls. One hundred and four of the genes could be classified as being involved in cell growth, cell death, or signal transduction. The results from 11 genes were selected for validation by quantitative RT-PCR. The average correlation coefficient was 0.655 (range 0.235–0.964).

Conclusion

This is the first analysis of global gene expression in MEN1-associated neuroendocrine tumors. Many genes were identified which were differentially expressed in neuroendocrine tumors arising in patients with the MEN1 syndrome, as compared with normal human islet cells. The expression of a group of apoptosis-related genes was significantly suppressed, suggesting that these genes may play crucial roles in tumorigenesis in this syndrome. We identified a number of genes which are attractive candidates for further investigation into the mechanisms by which menin loss causes tumors in pancreatic islets. Of particular interest are: FGF9 which may stimulate the growth of prostate cancer, brain cancer and endometrium; and IER3 (IEX-1), PHLDA2 (TSS3), IAPP (amylin), and SST, all of which may play roles in apoptosis.

Receptor-Mediated Tumor Targeting with Radiopeptides. Part 1. General Concepts and Methods: Applications to Somatostatin Receptor-Expressing Tumors

Radiolabeled peptides have become important tools in nuclear oncology, both as diagnostics and more recently also as therapeutics. They represent a distinct sector of the molecular targeting approach, which in many areas of therapy will implement the old "magic bullet" concept by specifically directing the therapeutic agent to the site of action. In this three-part review, we present a comprehensive overview of the literature on receptor-mediated tumor targeting with the different radiopeptides currently studied. Part I summarizes the general concepts and methods of targeting, the selection of radioisotopes, chelators, and the criteria of peptide ligand development. Then, the >400 studies on the application to somatostatin/somatostatin-release inhibiting factor receptor-mediated tumor localization and treatment will be reviewed, demonstrating that peptide radiopharmaceuticals have gained an important position in clinical medicine.

From basic to clinical research in gastroenteropancreatic neuroendocrine tumor disease -- the clinician-scientist perspective

Patients with rare tumors represent a diagnostic and therapeutic challenge for non-specialized physicians, surgeons and other medical doctors. Whereas several specialized centers have gathered data for an improved diagnosis and therapy of neuroendocrine tumor disease, numerous clinical issues have not been resolved on an evidence-based medicine level. Furthermore, the evaluation of new treatment options has been overshadowed by the low incidence of the disease. In this article, a major medical challenge for the diagnosis and therapy of neuroendocrine tumor disease is addressed. As well, new therapeutic treatment options translated from current findings in the fields of molecular and tumor biology are discussed.

Indium-111-labelled octreotide scintigraphy in the diagnosis and management of non-iodine avid metastatic carcinoma of the thyroid

Treatment of differentiated thyroid cancer is a success of modern medicine with the use of radioiodine ((131)I). However, a significant proportion of thyroid cancers may be non-iodine avid. Thyroid tumours are known to express somatostatin receptors. Octreotide, an analogue of somatostatin, can be combined with a radioactive isotope, such as (111)In-DTPA(0) to visualise tumours with high concentrations of somatostatin receptors. We assessed 18 patients with histologically proven metastatic or locally recurrent non-iodine avid thyroid carcinoma to determine the usefulness of (111)In-DTPA(0) octreotide scintigraphy compared to conventional radiology in diagnosing sites of metastasis. The diagnosis of metastatic disease was made using conventional radiology and all had prospective scintigraphy using (111)In-DTPA(0)octreotide. Of the 18 patients, 14 had octreotide-positive scans. In eight, the octreotide scans identified the same sites of metastases as conventional radiology, that is, were concordant. In nine patients, conventional radiology showed more extensive disease than revealed on the octreotide scans. In one patient with widespread bone metastases, octreotide gave a more detailed assessment of metastatic disease than conventional radiology. These data indicate that (111)In-DTPA(0)octreotide imaging for patients with non-iodine avid carcinoma of the thyroid may be a useful diagnostic and staging tool. One patient with Hurthle cell carcinoma metastatic to bone and a positive octreotide scan has been treated with (90)yttrium-labelled octreotide.

Optimising conditions for radiolabelling of DOTA-peptides with 90Y, 111In and 177Lu at high specific activities

DOTA-conjugated peptides, such as [DOTA(0),Tyr(3)]octreotide (DOTATOC) and [DOTA(0),Tyr(3)]octreotate (DOTA-tate), can be labelled with radionuclides such as (90)Y, (111)In and (177)Lu. These radiolabelled somatostatin analogues are used for peptide receptor radionuclide therapy (PRRT). Radioligands for PRRT require high specific activities. However, although these radionuclides are produced without addition of carrier, contaminants are introduced during production and as decay products. In this study, parameters influencing the kinetics of labelling of DOTA-peptides were investigated and conditions were optimised to obtain the highest achievable specific activity. The effects of contaminants were systematically investigated, concentration dependently, in a test model mimicking conditions for labelling with minimal molar excess of DOTA-peptides over radionuclide. Kinetics of labelling of DOTA-peptides were optimal at pH 4-4.5; pH <4 strongly slowed down the kinetics. Above pH 5, reaction kinetics varied owing to the formation of radionuclide hydroxides. Labelling with (90)Y and (177)Lu was completed after 20 min at 80 degrees C, while labelling with (111)In was completed after 30 min at 100 degrees C. The effects of contaminants were systematically categorised, e.g. Cd(2+) is the target and decay product of (111)In, and it was found to be a strong competitor with (111)In for incorporation in DOTA. In contrast, Zr(4+) and Hf(4+), decay products of (90)Y and (177)Lu, respectively, did not interfere with the incorporation of these radionuclides. The following conclusions are drawn: (a) DOTA-peptides can be radiolabelled at high specific activity; (b) reaction kinetics differ for each radionuclide; and (c) reactions can be hampered by contaminants, such as target material and decay products.

Anti-tumor effect and increased survival after treatment with [177Lu-DOTA0,Tyr3]octreotate in a rat liver micrometastases model

Peptide receptor scintigraphy with [(111)In-DTPA(0)]octreotide (a stabilized radiolabeled somatostatin (SS) analogue, OctreoScan) is widely used for the visualization and staging of somatostatin receptor-positive tumors. The application of likewise somatostatin analogues as vehicle for the deliverance of radionuclides to somatostatin receptor-positive targets are now in use for peptide receptor-targeted radionuclide therapy (PRRT). Currently preclinical and clinical investigation are ongoing trying to find the optimal combination of radionuclide and ligand. The anti-tumoral effects of such combinations, like [90Y-DOTA degrees, Tyr(3)]octreotide and [(177)Lu-DOTA degrees, Tyr(3)]octreotate, on SSR-positive solid tumors have been reported. In this study we present the anti-tumor effects of (177)Lu-DOTA-tate on: a) a single SSR-positive cell model and b) on a SSR-positive tumor in a rat liver micrometastatic model, mimicking disseminated disease. (177)Lu-DOTA-tate showed anti-tumoral effects in both cases and significant survival in the PRRT-treated rats. (177)Lu-DOTA-tate is a very promising new treatment modality for SSR-positive tumors, including disseminated disease.

Somatostatin receptor gene therapy combined with targeted therapy with radiolabeled octreotide: a new treatment for liver metastases

OBJECTIVE

To evaluate the effect of peptide receptor radionuclide therapy (PRRT) on somatostatin receptor (SSR)-transfected colon carcinoma cells in a rat liver metastases model.

SUMMARY BACKGROUND DATA

Previously the authors have shown highly effective therapy with PRRT of SSR-positive tumors. This treatment is SSR-mediated; successful treatment is seen only in SSR-positive tumors, with no effect in SSR-negative tumors. As many tumors lack this receptor, the idea arose to transfect SSR-negative tumor cells with an SSR gene to apply PRRT on these SSR-transfected tumor cells.

METHODS

CC531 colon carcinoma cells (SSR-negative) were transfected in vitro with an SSR (subtype 2) gene (CC2B). Liver metastases were produced after intraportal injection of these tumor cells in rats. On day 7, animals were treated with 185 or 370 MBq [177 Lu-DOTA0, Tyr3 ]octreotate. After 21 days rats were killed and liver metastases were counted.

RESULTS

Treatment with 370 MBq [177 Lu-DOTA0, Tyr3 ]octreotate showed a significant antitumor response in rats with CC2B liver metastases (SSR-positive) in comparison with controls. No significant antitumor effect was seen in PRRT-treated rats with CC531 liver metastases (SSR-negative). Also, a dose-dependent tumor response was seen in rats with CC2B liver metastases treated with 185 MBq [ 177Lu-DOTA0, Tyr3 ]octreotate compared with controls. In addition, rats with mixed liver metastases treated with 185 MBq [177 Lu-DOTA0, Tyr3 ]octreotate had significantly fewer metastases compared with controls.

CONCLUSIONS

The authors showed an impressive antitumor effect of SSR (subtype 2)-transfected colon carcinoma cells with PRRT in a rat liver metastasis model. Moreover, rats with mixed liver metastases had significantly fewer liver metastases compared with control rats, which may be due to a radiologic bystander effect of [177 Lu-DOTA0, Tyr3 ]octreotate. This phenomenon is beneficial in the concept of in vivo gene therapy.