Hydroxyethyl Starch: The Effect of Molecular Weight and Degree of Substitution on Intravascular Retention In Vivo

Quantitative analysis of degree of substitution/molar substitution of etherified polysaccharide derivatives

Due to the unique properties such as nontoxicity, biodegradability, availability from renewable resources, and cost-effectiveness, polysaccharides play a very important part in the science and technology field. The various chemically modified derivatives of these offer a wide range of high value-added in both food and non-food industries. Among the chemical modification, etherified polysaccharide is one of the most widespread derivatives by introducing an ether group which is commonly stable in both acidic and alkaline conditions. Hydroxyalkylation, alkylation, carboxymethylation, cationization, and cyanoethylation are some of the modifications commonly employed to prepare polysaccharides ethers derivatives. There also has been a growing tendency for creating new types of modification by combining the different means of chemical techniques. The correct determination of degree of substitution (DS)/molar substitution (MS) is crucially important. The objective of this article is to summarize developments in synthetic etherified polysaccharides, involving analytical methods for determination of MS/DS, measurement processes, and the associated mechanisms.

Investigating plasma volume expanders as novel macromolecular MRI-CEST contrast agents for tumor contrast-enhanced imaging

PURPOSE

The aim of this study was to investigate two clinically approved plasma volume expanders (dextran 70 and voluven) as macromolecular MRI-chemical exchange saturation transfer (CEST) contrast agents to assess tumor vascular properties.

METHODS

CEST contrast efficiency of both molecules (6% w/v) was measured in vitro at various irradiation saturation powers (1-6 μT for 5 s) and pH values (range, 5.5-7.9) and the exchange rate of hydroxyl protons was calculated. In vivo studies in a murine adenocarcinoma model (n = 4 mice for each contrast agent) upon i.v. injection provided CEST-derived perfusion tumor properties that were compared with those obtained with a gadolinium-based blood-pool agent (Gd-AAZTA-Madec).

RESULTS

In vitro measurements showed a marked CEST contrast dependency to pH, with higher CEST contrast at lower pH values for both molecules. The measured prototropic exchange rates confirmed a base-catalyzed exchange rate that was faster for dextran 70 in comparison to voluven. Both molecules showed a similar CEST contrast increase (ΔST% > 3%) in the tumor tissue up to 30 min postinjection, with heterogeneous accumulation. In tumors receiving both CEST and T₁ -weighted agents, a voxel-by-voxel analysis indicated moderate spatial correlation of perfusion properties between voluven/dextran 70 and Gd-AAZTA-Madec, suggesting different distribution patterns according to their molecular size.

CONCLUSIONS

The obtained results showed that both voluven and dextran 70 can be exploited as MRI-CEST contrast agents for evaluating tumor enhancement properties. Their increased accumulation in tumors and prolonged contrast enhancement promote their use as blood-pool MRI-CEST agents to examine tumor vascularization.

Controversies in the use of hydroxyethyl starch solutions in small animal emergency and critical care

OBJECTIVES

To (1) review the development and medical applications of hydroxyethyl starch (HES) solutions with particular emphasis on its physiochemical properties; (2) critically appraise the available evidence in human and veterinary medicine, and (3) evaluate the potential risks and benefits associated with their use in critically ill small animals.

DATA SOURCES

Human and veterinary original research articles, scientific reviews, and textbook sources from 1950 to the present.

HUMAN DATA SYNTHESIS

HES solutions have been used extensively in people for over 30 years and ever since its introduction there has been a great deal of debate over its safety and efficacy. Recently, results of seminal trials and meta-analyses showing increased risks related to kidney dysfunction and mortality in septic and critically ill patients, have led to the restriction of HES use in these patient populations by European regulatory authorities. Although the initial ban on the use of HES in Europe has been eased, proof regarding the benefits and safety profile of HES in trauma and surgical patient populations has been requested by these same European regulatory authorities.

VETERINARY DATA SYNTHESIS

The veterinary literature is limited mostly to experimental studies and clinical investigations with small populations of patients with short-term end points and there is insufficient evidence to generate recommendations.

CONCLUSIONS

Currently, there are no consensus recommendations regarding the use of HES in veterinary medicine. Veterinarians and institutions affected by the HES restrictions have had to critically reassess the risks and benefits related to HES usage based on the available information and sometimes adapt their procedures and policies based on their reassessment. Meanwhile, large, prospective, randomized veterinary studies evaluating HES use are needed to achieve relevant levels of evidence to enable formulation of specific veterinary guidelines.

The efficacy and safety of colloid resuscitation in the critically ill

Despite evidence from clinical studies and meta-analyses that resuscitation with colloids or crystalloids is equally effective in critically ill patients, and despite reports from high-quality clinical trials and meta-analyses regarding nephrotoxic effects, increased risk of bleeding, and a trend toward higher mortality in these patients after the use of hydroxyethyl starch (HES) solutions, colloids remain popular and the use of HES solutions is increasing worldwide. We investigated the major rationales for colloid use, namely that colloids are more effective plasma expanders than crystalloids, that synthetic colloids are as safe as albumin, that HES solutions have the best risk/benefit profile among the synthetic colloids, and that the third-generation HES 130/0.4 has fewer adverse effects than older starches. Evidence from clinical studies shows that comparable resuscitation is achieved with considerably less crystalloid volumes than frequently suggested, namely, <2-fold the volume of colloids. Albumin is safe in intensive care unit patients except in patients with closed head injury. All synthetic colloids, namely, dextran, gelatin, and HES have dose-related side effects, which are coagulopathy, renal failure, and tissue storage. In patients with severe sepsis, higher doses of HES may be associated with excess mortality. The assumption that third-generation HES 130/0.4 has fewer adverse effects is yet unproven. Clinical trials on HES 130/0.4 have notable shortcomings. Mostly, they were not performed in intensive care unit or emergency department patients, had short observation periods of 24 to 48 hours, used cumulative doses below 1 daily dose limit (50 mL/kg), and used unsuitable control fluids such as other HES solutions or gelatins. In conclusion, the preferred use of colloidal solutions for resuscitation of patients with acute hypovolemia is based on rationales that are not supported by clinical evidence. Synthetic colloids are not superior in critically ill adults and children but must be considered harmful depending on the cumulative dose administered. Safe threshold doses need to be determined in studies in high-risk patients and observation periods of 90 days. Such studies on HES 130/0.4 are still lacking despite its widespread and increasing use. Because there are safer and equally effective alternatives in the form of crystalloids, use of synthetic colloids should be avoided except in the context of clinical studies.