High relaxivity magnetic resonance imaging contrast agents. Part 1. Impact of single donor atom substitution on relaxivity of serum albumin-bound gadolinium complexes

Abstract

Rationale and objectives: The donor atoms that bind to gadolinium in contrast agents influence inner-sphere water exchange and electronic relaxation, both of which determine observed relaxivity. The effect of these molecular parameters on relaxivity is greatest when the contrast agent is protein bound. We sought to determine an optimal donor atom set to yield high relaxivity compounds.

Methods: A total of 38 gadolinium-1,4,7,10-tetraazacyclo-dodecane-N,N',N'',N'''-tetraacetato derivatives were prepared and relaxivity was determined in the presence and absence of human serum albumin as a function of temperature and magnetic field. Each compound had a common albumin-binding group and differed only by substitution of different donor groups at one of the macrocycle nitrogens. Oxygen-17 isotope relaxometry at 7.05 T was performed to estimate water exchange rates.

Results: Changing a single donor atom resulted in changes in water exchange rates ranging across 3 orders of magnitude. Donor groups increased water exchange rate in the order: phosphonate ∼ phenolate > α-substituted acetate > acetate > hydroxamate ∼ sulfonamide > amide ∼ pyridyl ∼ imidazole. Relaxivites at 0.47 and 1.4 T, 37°C, ranged from 12.3 to 55.6 mM(-1)s(-1) and from 8.3 to 32.6 mM(-1)s(-1) respectively. Optimal relaxivities were observed when the donor group was an α-substituted acetate. Electronic relaxation was slowest for the acetate derivatives as well.

Conclusions: Water exchange dynamics and relaxivity can be predictably tuned by choice of donor atoms.

Figure 1

A) Relaxivity of gadolinium complexes depends on a range of intrinsic molecular factors. B) For fast tumbling complexes, relaxivity is limited by fast rotation and it is not possible to determine how much relaxivity will be increased if rotation were slowed, e.g. by protein binding. C) Relaxivity is increased when the complex is immobilized by binding to a large protein, but other factors may now limit relaxivity.

Figure 2

General synthetic scheme for the compounds described in this report.

Figure 3

Chemical structures of the compounds studied in this report arranged by different donor groups.

Figure 4

A) Relaxivities determined in HEPES buffer, pH 7.4, 37 °C at 1.4T plotted versus compound molecular weight. B) Relaxivities determined in HSA solution at 37 °C showing the correlation between r₁ at 0.47T (y-axis) and 1.4T (x-axis). Squares represent suspected q = 2 compounds Gd6a-d; triangle is known q = 0 compound Gd3b; open circles are outliers Gd4a and Gd5c.

Figure 5

Relaxivities determined in HSA solution (0.1 mM Gd, 0.67 mM HSA) at 37 °C at 0.47T (black bars) and 1.4T (grey bars). A) Alpha-substituted acetate donors; B) Amide and N-heterocycle donors; C) all other donor groups.

Figure 6

A) Variable temperature NMRD profile for Gd1a in HSA solution. B) Variable temperature NMRD for q = 0 complex Gd3b in HSA solution. C) Estimated inner-sphere relaxivity of Gd1a obtained by subtracting each curve in B from corresponding curve in A. t = 35 °C (open circles); t = 25 °C (filled circles); t = 15 °C (open triangles); t = 5 °C (filled triangles).

Figure 7

Representative variable temperature (t = 35 °C (open circles); t = 25 °C (filled circles); t = 15 °C (open triangles); t = 5 °C (filled triangles)) NMRD with solid lines as fits to the data as described in the text. A) Gd4b; B) Gd1c; C) Gd7a; D) Gd2c

Figure 8

Reduced O-17 T2 relaxation rates versus reciprocal temperature (symbols) and fits to estimate water exchange parameters determined in the absence of HSA. A) Gd3c; B) Gd2g; C) Gd7a; D) Gd1a; E) Gd1c; F) Gd10a

Figure 9

A) Relaxivity of complexes in HSA solution at 37 °C plotted versus measured water residency time at 37 °C with data at 0.47T (filled symbols) and 1.4T (open symbols). Solid lines are curves generated using the mean molecular parameters listed in Table 1. B) Relaxivity of complexes in HSA solution at 37 °C plotted versus measured transient zero field splitting parameter Δ², a measure of electronic relaxation rate, with data at 0.47T (filled symbols) and 1.4T (open symbols). Solid lines are linear regressions.