Image Guidance: Interventions multiply for liver tumor therapy

Percutaneous options for hepatic tumor treatment are expanding, even as the incidence of primary and metastatic liver tumors continues to rise. While surgery remains the treatment of choice for patients meeting criteria for resection, results from minimally invasive treatments have exceeded those obtained with conventional chemotherapy or radiation, and it is possible that one or more of these techniques may soon vie with surgery as a treatment of choice for patients with liver tumors.^1-4

Experimental chemotherapeutic strategies, coupled with more liberal indications for lobar or segmental resection or even transplantation, are stimulating the development of novel percutaneous therapies. In many centers, the focus of metastatic tumor treatment has shifted from curative to goals that are more static or debulking in nature. Most of these antitumoral treatments are now performed using ultrasound guidance, and the role of US will likely expand as innovative treatments continue to be developed in research laboratories.

The expanded therapeutic uses of US for patients with hepatic malignancies include the following:

• ultrasound-guided drug, chemotherapy, or vector delivery;

• local delivery of an expanding variety of thermally mediated techniques; and

• promising conjoint therapies.

In the last, US is used to enhance the effects of other treatments. Several excellent reviews cover various aspects of these developments.

Drug Delivery

Direct US-guided delivery of antitumoral agents into hepatic tumors relies on the principle of direct tissue destruction from cytotoxic agents. High interstitial pressures within tumors oppose interstitial delivery of any drug, and tumor microvasculature requires improved delineation. The neovessels may serve as a barrier to drug delivery, but the well-documented leakiness of newly recruited tumor endothelial cells can also be used to enhance local deposition of macromolecular agents through endothelial pores (Figure 1).

No specific cell targeting occurs when these agents are injected directly into tumors, in part because of the composition of the cytotoxic agents being used. Several laboratories are attempting to synthesize agents that can be targeted to specific cell membranes by using monoclonal antibody conjugates. Antibodies can be bound to the surfaces of liposomes to enhance delivery of the vectors into tumors. Local delivery of ethanol produces immediate thrombus formation in all vessels, and this vascular occlusion can be used to enhance local delivery of any agents co-introduced with the alcohol. We have demonstrated this effect whereby ethanol not only causes destruction of tumor parenchyma but also serves to create a depot for the conjoint administration of fluorescent microparticles.

Ethanol's ease of use, limited morbidity, minimal technical requirements, and cost have made it the traditional agent for ablating intrahepatic tumors. Ultrasound is an ideal modality for guiding needle insertion and for monitoring directly delivery of the ethanol into the tumor (Figure 2). This technique has typically been reserved for cirrhotic patients with hepatocellular carcinoma-these tumors appear to possess a lower interstitial pressure than surrounding abnormal liver parenchyma and therefore retain the ethanol.^5-9

In contrast, liver metastases, such as those arising from colorectal cancers, have a higher interstitial pressure and cell density and do not retain as much ethanol. Indeed, US imaging of these tumors during ethanol administration frequently demonstrates brisk passage of ethanol into surrounding normal parenchyma. The role of US in these cases is to provide guidance during needle insertion and to document the targeted delivery of ethanol into the tumor. Ethanol appears brightly echogenic on US, and color flow can also be used to document or evaluate patency of nearby major vessels. To induce cell death throughout the tumor, ethanol must be delivered beyond the tumor periphery, and the images should document this process.

Research has focused on distinguishing necrotic or ablated tumor from residual viable tumor cells, and CT and MR have both been used to document enhanced perfusion to suggest the presence of residual viable tumor. However, ablation-induced angiogenesis and macrophage recruitment may lead to formation of granulation tissue that is indistinguishable from residual or even recurrent tumor. Targeted US contrast agents may play an important role in evaluation and follow-up of these tumors.¹⁰

Ultrasound is already clinically important in the targeted delivery of DNA or gene constructs into superficial tumors. Whether viral or nonviral delivery vehicles are employed, the technique essentially involves using US to deliver a small liquid aliquot into the center of a sonographically visible solid tumor. The ability to demonstrate this delivery in real-time with no radiation emphasizes the advantage of US over CT for this purpose.

A limitation of current ultrasound, CT, and MR contrast technology is the inability to image satisfactory gene insertion, uptake, and, ultimately, expression of desired gene products. Nuclear medicine is currently leading this field. Liposomal vectors can be synthesized to be sufficiently echogenic that their local delivery into a desired site can be imaged.

Thermal Techniques

A host of thermally mediated techniques are being used for ablating solid tumors.^1,11-20 Guided by US, the physician inserts needles percutaneously into solid tumors and ablates them using continuous and direct visualization. The choice of thermal technique depends in part on local expertise, availability of technology, and operator preference. Technical support and cost also influence the decision for physicians entering this expanding and competitive field.

Additional factors must be considered, as the physician decides whether these procedures will be performed percutaneously, using laparoscopic US guidance, or as open procedures in the OR. These decisions are influenced by the types of tumors being treated, extent of involvement of referring surgeons, and extent of disease.

Radio-frequency ablation is being used in multiple centers.^11-13 Thermally shielded needles are introduced percutaneously into a tumor under US guidance, and US is used to monitor the extent of tumor destruction. The zone of coagulative necrosis is brightly echogenic, and we have shown that this results from formation of microbubbles in the destroyed tissue.

It is important that the zone of ablation extend into peritumoral normal liver to optimize the extent of tumor kill. Ultrasound is particularly helpful for documenting extension of the echogenic zone into peritumoral tissue along all margins of the spherical tumor. It is also used to document proximity to major vascular and extrahepatic structures.

Interstitial laser photocoagulation¹⁴ and microwave ablation also produce zones of echogenicity throughout the area of coagulative necrosis, presumably due to formation of microbubbles. In contrast, laparoscopic, open, or percutaneous cryoablation^15-18 produce a rounded echogenic mass corresponding to the iceball. This iceball is visualized as an expanding, echogenic, hemispherical rim, which produces acoustic shadowing that may obscure parts of the tumor. Care must be taken to image the lesion from different locations to ensure that adequate margins are obtained, and US is used to document that the freeze zone extends beyond the margins of the tumor being ablated (Figure 3). Since cryoablation frequently involves use of alternating freeze-thaw cycles, US is used to document the changing appearance of the iceball during these cycles.

High-frequency focused US has been used for the local induction of hyperthermia.¹⁹ Numerous biological and clinical investigations have demonstrated that hyperthermia in the 41° to 45° C range can enhance clinical responses to radiation therapy and has potential for enhancing other therapies, such as chemotherapy, immunotherapy, and gene therapy. Furthermore, high-temperature hyperthermia (greater than 50° C) alone is being used for selective tissue destruction as an alternative to conventional invasive surgery.

Hynynen has demonstrated that sustainable arterial occlusion can be induced by focused US energy deposition noninvasively introduced within deep tissue.20,21 While not yet widely accepted or used, this US technology may have important potential applications for enhancing antitumoral therapies in the liver.

Conjoint Approaches

An expanding area of great promise for US technologies is targeted or conjoint drug delivery, whereby US enhances and promotes the local delivery of desired chemotherapeutic drugs into tumors or specific anatomic sites. Sonographic contrast agents can be designed not only to target specific anatomic locations or compartments but also to liberate their aqueous contents following rupture by a focused US beam.

Langer²² first described the ability of US to release incorporated molecules from solid polymers. His work represents the foundation for the entire therapeutic potential of US-enhanced delivery of incorporated substances. Langer's group has shown increases in release rates that are proportional to the intensity of the US beam.²³

The ability to control internal drug activity is being actively explored by several groups. Point Biomedical has developed a double-walled microsphere in which the individual wall components can be independently modified to optimize imaging properties, such as sensitivity, resolution, and fragility. The outer wall can be modified to prolong intravascular residence time or to target specific cell surface receptors. Perhaps even more innovative is the ability to engineer the internal wall to respond to specific US frequencies. This precise control allows the microspheres to be used as drug delivery vehicles; enzymes and a range of different-sized molecules have been successfully inserted. The acoustic properties of these microspheres can be modifed to respond to an ultrasound beam.

Already, exposure to US signals induces rupture of the microspheres, releasing contained therapeutic molecules lying as much as 20 cm below the skin surface. It is foreseeable that encapsulated drugs will be delivered intravenously, targeted to specific anatomic or receptor sites by monoclonal antibodies or peptides conjugated to the outer membrane. Subsequent US-triggered release of the aqueous components could reduce possible systemic side effects of some chemotherapeutic drugs.

Ultrasound-visible microspheres conjugated to ligands for specific molecular epitopes selectively adhere to biological surfaces expressing that epitope. As an example, prior to the development of atherosclerosis, endothelial cells express adhesion molecules that mediate monocyte adhesion during atherogenesis. Monoclonal antibodies directed toward one of these endothelial receptors, ICAM-1, have been conjugated to the outer surface of lipid-derived microspheres.²⁴ These microspheres, filled with a perfluorobutane gas, are visible on US and bind specifically to upregulated endothelial cells. Ultrasound therefore has the potential to characterize cell phenotype in vivo.

Improvements in the design and synthesis of US contrast agents permit the evaluation of contrast agents as drug-carrying vectors. Not only can some of these vectors be visible in vivo, but conjugation of monoclonal antibodies to the surface of liposomes is used to further target US contrast agents to specific sites in the body. It is entirely feasible that US will be used to confirm the delivery of echo-visible vectors to desired anatomic or physiological sites, liberate the aqueous contents, drive these out of the vascular pool and into cells, and ultimately enhance the uptake of DNA into cells, stimulating transfection and expression.

Bednarski and colleagues²⁵ have explored the role of US in enhancing targeted delivery of macromolecules. Using MR-visible liposomes, this group has shown that pulsed-focused US increases endothelial permeability for liposomes. Using energy values below those for theoretical thermal damage, US altered the endothelium to permit passage of 100-nm macromolecular structures. These liposomes contained aqueous MR contrast materials, emphasizing the potential ability of microspheres to deliver therapeutic agents to specific sites (Figure 4).

Skyba and colleagues²⁶ have shown that application of US to thin-shelled microbubbles flowing through microvessels produces vessel wall rupture in vivo. When microbubbles are exposed to US, their destruction causes microvessel rupture large enough to permit extravasation of red blood cells.²⁷ Additional work should document the extent and rates of reversibility of the endothelial changes, the particle sizes that can be displaced through vessel rupture sites, and the effects of different US frequencies and durations on endothelial pore and rupture size.

Ultrasound-guided sonoporation-high-frequency waves opening membrane micropores and making cells temporarily permeable to small molecules-can be used to enhance drug delivery into cells. By applying appropriately timed pulses and levels of energy, US may be used to enhance uptake of DNA into cells and also to induce transfection and ultimately expression of desired gene products. Manome et al demonstrated that mechanical insonation with US facilitates transduction of naked plasmid DNA into colon carcinoma cells in vivo.²¹ The local mechanical changes induced by the US-produced hyperthermia may increase endothelial permeability, providing an additional means by which sites can be targeted with intravascular macromolecular agents.

Enthusiasm for gene therapy and genomics is tempered by the lack of safe and efficient techniques for delivering genes or DNA constructs into cells and confirming their successful delivery. Ultrasound may play several important roles in different components of gene therapy. We have described the ability of US to image labeled microspheres at target sites and to enhance delivery of macromolecules beyond the endovascular compartment. Attention is also being focused on the ability of US to enhance gene expression.28 Several groups have shown that 1 MHz of continuous-wave therapeutic US after liposome-mediated transfection of cultured cells enhances expression of gene products.^28-31 Clearly, the combined ability of US to deploy gene-carrying vectors to target sites and to enhance the liberation, release, and ultimate expression of desired products suggest that it will play a pivotal role in the developing technologies associated with human gene therapies.

Dr. Kruskal is an associate professor of radiology at Harvard University, and director of the abdominal imaging training program at Beth Israel Deaconess Medical Center in Boston. Dr. Kane is a professor of radiology at Harvard and director of ultrasound at Beth Israel Deaconess.

References

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