In general, uptake in the lungs does indicate formation of colloid. This formation is probably not in vivo, but occurs during or after the synthesis/formulation (but before injection).

The cause may be either bad solubility or the formation of a complex that is less soluble. There is a paper discussing the tracer (http://www.sciencedirect.com/science/article/pii/S0969804308004363) that mentions colloid formation is possible and how to test for it, but they do not describe how the colloid is formed.

How to avoid it: probably by using a suitable buffer that allows complexation with the buffer and still remaining soluble. The phosphate functions are prone to precipidation with a number of salts (calcium,...) so perhaps it is a question of avoiding the wrong buffer.

I hope this helps,

Matthias

particle needs to be larger than colloidal to be trapped in the lung. A colloid would be expected to be trapped by the reticuloendothelial cells of the liver, spleen, and bone marrow.

When using TLC to identify radio-chemical purity of complex, its more than 99% which suggest that there is no formation of colloidal in the synthesis of complex and colloidal formation occur after injection.

Lu177-ZLD formulation accomplished using Phosphate buffer and also with out phosphate buffer, the results in both cases are similar. so, the question arises which buffer must be used to increase the solubility of the complex.

@Edward Russak , The particle size is larger than colloidal and its trapped in the lung rather than staying in the Reticuloendothelial cell of liver , spleen and bone marrow.

you could try to pass the mixture of a molecular weight cut-off filter unit (take cut-off value to be 1.5 times the actual molecular weight). If any form of colloid (large or small, yes that will impact the site where it sticks) is present, you should pick it up there.

If it passes through, then perhaps it just really binds there?

Suppose there is formation of colloidal during formation of complex (Lu177-ZLD), an what ever the size of colloidal. How would I avoid colloidal formation? I have used phosphate buffer of pH=8 ( sodium dihydrogen carbonate and disodium hydrogen carbnonate).

Sorry, thought this was observed in vivo

Difficult question you ask there Nadeem. A Hepes buffer perhaps, mayby different pH conditions (pH 6 instead of 8), mayby some albumine to help keep it soluble. There is no one answer I'm afraid.

(and, in addition, it may be that it just binds to something in the lungs)

According to all of my training and all of my textbooks, particles of colloidal dimensions are too small for accumulation in the lung. In Nuclear Medicine studies, a radiolabeled sulfur-based colloid is used to visualized the liver, spleen, and bone marrow. Radiolabeled macroaggregated albumin in the form of a suspension is generally used for lung imaging. These particles are much larger than those found in colloidal dispersions. The albumin particles can be visualized using a simple light microscope, and may settle upon standing. A true colloid, such as smoke, ink, or homogenized milk will not settle on standing.

you are correct edward. "big" colloids (one can question if they should be called colloids, as macroagreggates is a better name) go to the lungs, while smaller colloids go to liver/spleen.

It's up to Nadeem to experiment however, and see what type he has (if any)

You both are correct, while preparing the 177Lu-ZLD complex, I can not see any formation of colloidal nor the formation of macroagreggates. They formation of colloidal or macroagreggates can be seen naked eyes, and we can guess through the turbidity of solution.

But in my case i obtained the clear solution during formation of complex. Is it possible that macroagreggates formation occur in vivo? This factor is also eliminated when i took the blood sample from injected rabbit and analyzed through biological microscope, red blood cell are in good shape and cell morphology is not changed.

what are the factors which make Lu177-ZLD to reside in the lungs>???

Like your response Matthias. Perhaps some metabolic transformation post-administration is responsible? as Nadeem suggests. I wonder if any autoradiographic techniques might elucidate this mechanism or at least suggest exact location in lung tissue this species is located.

If you use autoradiography of (cryo)microtome slices, you should be able to tell of the activity is in the vessels, or in the lung itself.

I have experimented again and see there is formation of microagreggates upon heating of sample to around 40C. Without heating there is no formation of microagreggates a clear solution of 177Lu-Zoledronic acid is obtained. I think when injected IV in the ear vein of rabbit, due to the temperature of blood the complex form microagreggates and accumulate in lungs, although some of the activity retained in the skeletal which is my required interest.

Now the question rises how to avoid the formation of microagreggates of the complex.

One possible solution: make your own controllable precipitation: mix your Lu-zoledronic acid with a few mg (rabbit) serum albumine at 35C. Once it starts to precipitate, it may bind to the albumine and remain soluble that way.

Offcourse, this may alter your pharmacokinetics, but "slower" is better then "wrong".

Other options: ethanol to increase solubility.

One question though: if your compound formes macroagreggates at 40C, but not 20C, I am worried that it is unstable and that perhaps your side products form the agreggates. Can you check stability and see if there is anything else besides the parent compound and the agreggates?

Zoledronic acid is very stable and form stable complex with lanthanides, some results about the 177Lu-Zoledronic acid has already been published, showing its potential as palliation for bone pain metastases.

I have conducted SPECT imaging on rabbit for being taken up by bone (skeletal) but it also showed lungs uptake.

I would try ethanol to increase the solubility of complex.