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Eur. Radiol. 5, S 107-S 111 (1995) © Springer-Verlag 1995

Looking into the future: iotrolan and beyond H. P. Niendorf, T. Fritzsch, G. Herrschaft, W. Krause, A. Miihler, R. Schiirmann

Clinical Research and Development Diagnostics, Schering AG, Mtillerstrasse 178, D-13 353 Berlin, Germany

This paper will give a personal overview of the current status of the development of contrast agents for X- ray, MRI and ultrasound imaging, and will suggest the types of agent we may be using within the next few years. Examples of agents presented in this review include those of which the authors have personal experience.

Introduction

Throughout the history of contrast agents for medical imaging, the development of new agents has been moti- vated by the pursuit of the highest degree of safety. The "ideal" contrast agent - effective, but with no clinically significant effects on any biological systems - has still to be developed, but modern agents used for enhancement of X-ray, magnetic resonance imaging (MRI) and ultrasound imaging are all considered to be very safe. Nevertheless, research is continuing, with the main thrust now towards improving the specificity of agents to particular tissues while maintaining the highest standards of safety achieved with today's contrast agents.

X-ray contrast agents

From their earliest beginnings up to the present, the safety and tolerability of X-ray contrast agents has been steadily improved. The binding of iodine to ben- zoic acid derivatives marked the start of the era of "modern" agents, and a milestone was reached with the development of non-ionic compounds. Since this significant leap in the safety standards of these agents, additional improvements in safety are increasingly difficult to achieve. The recent development of the non-ionic dimeric agents is possibly the final stage of this evolution, with little indication at this time that further safety benefit will be achieved from further work with this sub- stance class.

The safety and tolerability of a contrast agent de- pends mainly on two factors: first, osmolality, and sec- ondly, chemotoxicity, to which all non-osmolality- related effects, e.g. lipophilicity, high protein-binding and the degree of histamine release, contribute. Non- ionic monomers showed substantially reduced protein- binding and histamine release in comparison with ionic agents, thus exhibiting significant safety advantages [1]. However, it is with the non-ionic dimers that hydrophili- city is at its greatest, and the goal of blood isotonicity at useful iodine concentrations has been reached. Formu- lations of iotrolan, the first non-ionic, dimeric X-ray contrast agent (Isovist ®, Schering AG, Berlin, Ger- many) are blood isotonic at iodine concentrations of up to 320 mg I/ml.

Correspondence to: H. R Niendorf

S 108 H. E Niendorf et al.: Iotrolan and beyond

Iotrolan

In 1988, iotrolan was introduced in formulations of 240 mg I/ml and 300 mg I/ml for use in lumbar, thoracic and cervical myelography, including ventriculography and cisternography, and for the opacification of other open and closed body cavities.

Myelography is a particularly sensitive indication and, as such, provides a significant test of the safety and tolerance of this new agent. By the end of 1993, over 1.5 million examinations using the 240 or 300 concentrations, in more than 20 countries worldwide, had demonstrated both the exceptional safety and efficacy of iotrolan [2].

The new 280 mg I/ml formulation has been developed to take the unsurpassed safety profile of iotrolan to all X-ray investigations requiring intra-arterial or intravenous contrast agent administration. The 280 mg I/ ml concentration combines excellent safety with both high quality contrast density and ease of administration. At this time, iotrolan has been approved in Ger- many, Sweden, Belgium, Denmark and The Nether- lands, Japan and China, has been submitted to most Eu- ropean countries, and is undergoing phase III clinical trials within the USA.

Phase II studies using iotrolan 320 in angiocardio- graphy have been completed. Results suggest that lar- ger clinical studies, and studies in specific, well-defined patient groups, will demonstrate the anticipated advantages of this concentration: reduced influence on the he- modynamics of the heart, particularly blood pressure and left ventricular end diastolic pressure, and mini- mized electrocardiography changes in comparison to non-ionic monomers.

Future developments in X-ray contrast agents

For the past 60 years, safety has been the prime driving force behind contrast agent development and the characteristics of safety and tolerance have been improved steadily. The step from ionic to non-ionic contrast agents proved to be an important one, because with the non-ionics an unprecedented high safety standard was reached. Describing the development of contrast agent safety as a curve (Fig. 1) we see an early, steeply ascend- ing portion indicating the development from ionic to non-ionic agents, and a reduced slope representing the development from non-ionic, low-osmolar to non-ionic, iso-osmolar contrast agents. The dimeric, isotonic iotrolan lies on the asymptotic final stage of this safety curve; i.e. the maximum achievable safety in this sub- stance class has probably been reached. It is hard to imagine how the safety of this class of agents can be significantly improved any further.

If there is a demand for safety at a level beyond that now achieved, one may have to switch over to entirely different compounds. One group of agents that may offer additional safety benefits are the metal chelates such as gadolinium-DTPA (Gd-DTPA, Magnevist ®, Schering AG, Berlin, Germany). Gadolinium-DTPA,

>, g

Metal chelates Non-ionic Non-ionic __.._________~isotonic (e.g. Gd-DTPA)

low o s m o ~ E s =1-2%

°sm?=10-15%

1953 1981 1988 1992

Fig. 1. The increasing safety of X-ray contrast agents. *AEs, Ad- verse events

as discussed later, has an exceptional safety profile when used for MRI [3]. Investigations using this agent for intra-arterial DSA [4] and for computed tomogra- phy (CT) [5] have shown the validity of this approach. Further research into the potential of Gd-DTPA as an X-ray contrast agent seems to be promising for special patient groups, for example, patients with hyperthyroid- ism or with known allergy-like reactions to iodinated X- ray contrast agents.

Having reached the goal of a high safety profile of X- ray contrast agents, the main thrust of future contrast agent research and development will, therefore, be fo- cused on improved efficacy, that is the specificity, of new agents. Novel patterns of biodistribution and excretion will offer many advantages in terms of organ- or tissue- specific investigations.

The most promising avenues are hepatocyte-targeted and reticulo-endothelial system (RES)-targeted contrast agents, the latter permitting contrast enhancement of the liver and spleen. These agents distribute both ex- tracellularly and intracellularly, and are submitted to ac- tive uptake by the cells. In this context, blood-pool contrast agents may play an important role in contrast-enhanced perfusion studies and as markers of regions of pathologically increased capillary permeability in the body.

Iopromide-carrying liposomes

Iopromide (Ultravist ®, Schering AG, Berlin, Germany) is a non-ionic, monomeric, iodinated contrast agent with an excellent tolerance and safety profile that has been confirmed in extensive clinical use since 1985.

Encapsulation of this agent within liposomes repre- sents an innovative approach in the field of X-ray contrast agents and leads to a pronounced modification of the distribution pattern of the agent. Whereas iopromide acts as an extracellular contrast agent, iopromide li-

H. R Niendorf et al.: Iotrolan and beyond

posomes are taken up mainly by the Kupffer cells of the liver and the macrophages in the spleen, resulting in an accumulation of contrast within the liver and spleen following intravenous injection [6, 7]. Once within the cells, the liposomes are broken down and their constitu- ents are metabolized; iopromide is not metabolized but is released by the phagocytes and excreted by the renal route [8].

This preparation shows promise for the differentia- tion of tumor and healthy tissue. Tumor tissue has no, or only limited, phagocytic activity so contrast will be selectively concentrated in the healthy tissue of the liver, for example. In rabbits with implanted VX-2 tumor, CT imaging using iopromide liposomes was able to delineate liver lesions with a diameter of well below 1 cm.

This evidence suggests that iopromide liposomes may become an important tool for increasing the detection of focal liver lesions. As the liver is the organ most frequently involved in secondary metastases of primary gastrointestinal lesions, the ability to accurately detect secondary focal liver tumors improves their staging and may provide information necessary for a well planned treatment. The biodistribution of the iopromide liposomes may also enable the detection of focal lesions in the spleen.

In addition to X-ray procedures, MRI and ultrasound imaging represent important diagnostic imaging techniques. With increasing use of these newer procedures, the clinical need for specific contrast agents has also increased.

Contrast agents in magnetic resonance and ultrasound imaging

The development of contrast agents for the three diagnostic areas of X-ray, MRI and ultrasound, differed in several respects. X-ray agents were developed over de- cades until the desired safety profile was achieved, whereas the first MRI agent, Gd-DTPA, surpassed the safety profile of even the safest X-ray contrast agent [3]. As with the X-ray agents, MRI contrast agents are now being developed with organ specificity and blood- pool characteristics.

M R I contrast agents

Gadolinium-DTPA, a highly stable metal chelate, first received clinical approval for use in cranial and spinal MRI during 1988, and has since been approved, almost worldwide, for whole body indications.

The molecule is small and highly hydrophilic, and so is rapidly distributed throughout the entire extracellular space, with the exception that it cannot cross barri- ers such as the intact blood-brain barrier. Thus, Gd- DTPA is highly sensitive for the detection of impair- ment of this barrier, for example as a consequence of tumor growth or inflammatory disease. However, this also means that to achieve dynamic studies outside the cen-

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tral nervous system (CNS), first pass imaging with pre- cise indication-specific timing is essential.

For whole body indications, Gd-DTPA has proven to be especially useful in the dynamic studies for diagnosis of breast carcinoma and osteosarcoma as well as lesions of the kidneys, adrenal glands and the liver [9]. Dynamic studies using Gd-DTPA have recently provided Harem et al. with interesting results for the differential diagnosis of focal liver lesions [10].

In response to the clinical need for an MRI contrast agent that tends to remain within the vascular space, i.e. a blood-pool agent, the potential of Gd-DTPA- polylysine, a derivative of Gd-DTPA, is now undergoing preclinical evaluation. If successful, this agent should greatly enhance the ability to distinguish blood vessels from the surrounding tissues and extracellular space.

Gadolinium-DTPA-polylysine combines the proven efficacy and safety of Gd-DTPA with a high molecular- weight backbone of polylysine. Even though the com- pound is not strictly confined to the vascular space, the half-life of distribution into the extracellular space is significantly prolonged due to its high molecular weight, resulting in much higher vascular concentrations of this new gadolinium complex. Consequently, a very low dose of Gd-DTPA-polylysine, such as 20 ~tmol Gd/kg, is sufficient for improving MRI angiograms. Promising results have also been achieved during perfusion studies, particularly of the heart. In a way analo- gous to Gd-DTPA in the CNS regarding the blood- brain barrier, the status of capillary permeability in other body regions can be assessed, with potential for the detection of occult bleeding and areas of increased capillary permeability.

Like Gd-DTPA, the polylysine derivative is cleared almost exclusively by glomerular filtration, although elimination times are prolonged [11]. No biodegrada- tion or metabolism has been observed, and only trace quantities of Gd 3 ÷ can be measured at 7 days post-injection [12].

Magnetic resonance imaging of the liver is an important tool, but suffers from problems such as motion artefacts and the lack of liver-specific contrast agents. Tur- bo-FLASH imaging sequences have been developed to avoid artefacts, but the problem of the lack of a specific contrast agent remains. The benefits to be gained poten- tially include increased sensitivity to mass lesions while avoiding the complex acquisition sequences presently used.

Liver-specific contrast enhancement is achieved using another Gd-DTPA derivative, Gd-ethoxybenzyl (EOB)-DTPA. The lipophilic EOB moiety, covalently linked to Gd-DTPA, alters the pharmacokinetics and biodistribution of the agent in vivo. Via the organic transport system, Gd-EOB-DTPA is taken up by hepa- tocytes and excreted via the bile into the faeces. This agent is eliminated from the body by both the renal and hepatic route independent of dose and in roughly equal proportions.

Gadolinium-EOB-DTPA acts by markedly reducing the T1 relaxation times in the liver. The detection

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threshold of focal liver lesions is expected to be well below i cm in diameter. Ongoing phase II studies confirm the expected high tolerance and excellent contrast quality, and have shown that the dose necessary to achieve diagnostically significant contrast enhancement is dis- tinctly lower than Gd-DTPA; 25 ~tmol/kg may be ade- quate for diagnostic purposes.

The biliary excretion of Gd-EOB-DTPA creates the possibility for new indications. Excretion of the agent into the biliary system, and subsequently the bowel, may permit functional and morphological assessment of biliary drainage as well as the delineation of duode- nal structures [13].

SH U 555 A is a stable, hydrophilic solution of super- paramagnetic iron oxide particles coated with 2 nm layer of carboxydextran. The iron oxide core consists of a crys- talline nanoparticle (mean diameter 4 nm) composed of a mixture of Fe304 (Magnetite) and y-Fe203 (Maghemite). SH U 555 A is an RES-targeted agent that, following intravenous administration and subsequent uptake in normal spleen and liver tissue, shortens predominantly the T2 relaxation time, thus producing a pronounced signal loss, particularly on T2-weighted images.

Tumor tissue can be detected as it contains either no RES or a disturbed RES and cannot, therefore, incorpo- rate the contrast agent as effectively as healthy tissue. This produces a contrast between healthy and tumor tissue that can lead to the detection of very small lesions.

During a phase I study, SH U 555 A showed very good tolerance even after intravenous bolus injection at a rate of 500 ~mol Fe/min [14]. Due to the extensive safety studies performed, initial MR images were obtained as late as 8 h post-injection, and these showed significantly decreased signal intensities in the liver and spleen. Doses as low as 8 ~mol Fe/kg are sufficient to significantly reduce signal intensity.

Phase II studies are ongoing. Results obtained to date suggest that maximal contrast is achieved after as little as 40 min. The long-lasting enhancement (effective between 40 min and 8 h) may represent an advan- tage regarding flexible examination timing. In addition, fast intravenous injection of SH U 555 A shows the potential for evaluation of the perfusion of intrahepatic lesions during the distribution and retention phase of the contrast agent.

Ultrasound echoenhancing agents

As with MRI contrast agents, the first ultrasound echoenhancing agent, SH U 454 (Echovist ®, Schering AG, Berlin, Germany) demonstrated an exceptional safety profile. However, unlike contrast agents for X-ray and MRI, both SH U 454, and its successor, SH U 508 A (Levovist ®, Schering AG, Berlin, Germany) produce signals strictly from within the vascular bed and thus produce an angiographic effect. Further developments of ultrasound echoenhancing agents are resulting in organ-specific agents.

SH U 454 itself consists of galactose microparticles which, shortly before use, are shaken with an aqueous


galactose solution. The resulting suspension contains galactose microparticles with adhering air bubbles in the micrometer range. Following intravenous injection, this suspension increases the usually low echogenic properties of the blood to a large extent.

The galactose microparticles and microbubbles of SH U 454 rapidly dissolve into the blood plasma, and cannot cross, via the lungs, from the venous to arterial phases of the circulation. Consequently, SH U 454 is only suitable for right-heart echocardiography and has been approved and launched in this indication in 1991 in Germany. Diagnostic efficacy and tolerance has also been proven in extended clinical trials for the indications of phlebosonography and hysterosalpingo contrast sonography and those indications have been submitted for approval to the health authorities of several countries.

SH U 508 A is similar to SH U 454 except that it in- cludes 0.1% palmitic acid and is suspended in pure wa- ter. The microbubbles released from this new formulation are stabilized by the palmitic acid; they display much greater intravascular stability and are able to sur- vive pulmonary transit to achieve left-heart opacification, improving, for example, endocardial border delineation, and furthermore reach the arterial vascular bed.

The signal-to-noise ratio of Doppler sonography can be improved significantly with SH U 508 A, and the improved sensitivity of color Doppler flow opens up new diagnostic capabilities, such as the reliable vascularity imaging of tumor lesions [15]. The improved detectabil- ity of tumor vessels e. g. in breast carcinomas, has led to improved tumor characterization [16]. In studies using color Doppler of the internal carotid artery, SH U 508 A at 200 mg/ml may be most useful for the diagnosis of stenosis or occlusions of the neck arteries in patients who exhibited non-diagnostic Doppler signal intensity during routine examination [17].

The first liver-specific ultrasound echoenhancing agent is SH U 563 A, which consists of gas-filled biopolymer spheres with a mean diameter of 1 ~m and a wall thickness of about 50 nm. These microspheres are small enough to pass through lung capillaries and stable enough to retain their echoenhancing properties for a prolonged period.

Targeted to cells of the RES, SH U 563 A distributes differently between normal liver structures and tissues not containing Kupffer cells (e.g. neoplastic tissues). Consequently, in animal experiments it leads to demar- cation of tumors. Phase I studies are planned to begin in the near future.

Summary

The safety and tolerance of X-ray contrast agents, the driving force in their evolution over the past 60 years, have now reached a satisfactory level, and new avenues are being explored. Blood-pool and organ- or tissue- specific agents are likely to improve diagnostic confi- dence in a range of indications where conventional agents are less suitable. Magnetic resonance imaging


and u l t rasound agents show a similar t rend, but have the advan tage that the first agents met the safety s tandards expec ted by today ' s clinicians.

The d e v e l o p m e n t of cont ras t agents for all imaging techniques is one of the mos t rapidly expand ing fields in biological research, and new agents are very excit- ing; they p romise to fulfil the dual needs of a high safety profi le and the eff icacy necessary for conf idence in diagnosis.

References

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