Indicator compounds, method of their preparation and use of those compounds in an iron assay system

The present invention is related to compounds of the general formula (I) ##STR1## wherein R.sup.1 denotes hydrogen, halogen or C.sub.1 -C.sub.4 alkyl and PA1 R.sup.2 denotes C.sub.1 -C.sub.4 alkyl, aryl or heteroaryl, or a substituted C.sub.1 -C.sub.4 alkyl, aryl or heteroaryl and their preparation. The compounds can be used in an iron assay system.

SUMMARY OF THE INVENTION 
The present invention is related to compounds of the general formula (I) 
##STR2## 
wherein R.sup.1 denotes hydrogen, halogen or C.sub.1 -C.sub.4 alkyl and 
R.sup.2 denotes C.sub.1 -C.sub.4 alkyl, aryl or heteroaryl, or a 
substituted C.sub.1 -C.sub.4 alkyl, aryl or heteroaryl, and their 
preparation. The compounds can be used in an iron assay system. 
Alterations of iron metabolism reveal themselves both as deficiency 
diseases and overload diseases of the element. Iron deficiency diseases 
are widely diffused and also affect populations of highly developed 
countries besides obviously underfed people. Iron deficiency anemia 
preferentially affects women for increased iron losses with menstruation, 
pregnant women for increased iron needs, patients with ulcer or other 
causes of acute or chronic bleeding. Also neonates are particularly 
exposed to the disease. 
The diagnosis of iron deficiencies, their differentiation from low blood 
iron during phlogistic diseases, and the monitoring of therapy, require 
accurate and reproducible laboratory tests. 
The laboratory diagnosis in this field takes advantage essentially of the 
measurements of: 
blood iron 
blood transferrin 
blood ferritin 
globular volume and hemoglobin 
free erythrocyte protoporphyrin. 
While all the other tests are largely established in the clinical 
laboratory, the free erythrocyte protoporphyrin assay is now emerging from 
the "experimental incubation" phase for the clinical routine application. 
The term blood iron refers to the level of iron actually transported in the 
plasma and consequently bound to transferrin. Although it is not a sure 
index of the iron content of the body, the blood iron assay is valuable to 
estimate the status of the amount of stored iron. Major causes that can 
lead to an iron deficiency or surplus are shown in Table I. 
TABLE I 
______________________________________ 
Causes of Variation of Blood Iron 
______________________________________ 
CAUSES OF DECREASE 
Insufficient intake of dietary iron 
(babies, vegetarians) 
Defective absorption 
(total and subtotal gastrectomy, achlorhydria, 
chronic diarrhoea and steatorrhoea) 
Prolonged blood losses 
(chronic hemorrhage due to gastric, duodenal 
ulcer, etc.) 
Increased needs 
(pregnancy, lactation) 
Iron storage in the cells of the RE system 
(chronic or other infections) 
CAUSES OF INCREASE 
Increased degradation of erythrocytes 
(Hemolytic anemia, autoanticorpal anemia) 
Disorders of hemoglobin synthesis 
(pernicious anemia, sideroachrestic anaemia) 
Acute liver diseases 
(viral hepatitis, toxic hepatitis) 
Hemosiderosis 
Hemochromatosis 
______________________________________ 
Iron deficiency generally evolves through various phases slowly before 
resulting in frank anemia. It should be noted that a low blood iron level 
does not necessarily reflect the existence of a status of iron deficiency. 
Blood iron in addition deviates markedly from normality only when the 
variations of the status of saturation of the amount of stored iron have 
become significant. 
The blood iron is A quite variable parameter. It presents marked and well 
known fluctuations both within-a-day and between-days. 
Various authors have documented the existence of a circadian rhythm of 
blood iron, with a peak in the morning between 8 and 10 and lower values 
in the late afternoon. Particularly interesting is that, in subject 
working at night, the more elevated values are shifted in the afternoon, 
in phase with the cycle-sleep-activity; consequently the rhythm appears to 
be reversed. 
The reference intervals reported in the literature are also different; the 
normal values are, among other things, influenced by physiological factors 
such as age (higher values in the neonate and lower values in the elderly) 
and sex (slightly high values in men). 
The biological variability of blood iron and the possibility of increases 
for cellular necrosis processes (for example acute liver diseases) and 
decreases for phlogistic conditions (because of the bond with transferrin) 
limit the diagnostic value of the measurement. 
For more detailed information reference is made, for example, to "Iron 
Clinical Significance and Methods of Assay" a publication of July 1986 
edited by Ames Division, Miles Italiana S.p.A. 
The most serious problem of iron testing is the low concentration of the 
analyte especially in case of a decrease. Further the tight bond between 
the iron and the transferrin requires drastic reaction conditions (in the 
assay system) to release the iron from this transport protein. 
Interference caused, for example, by copper is a serious problem, too. 
The present invention now provides new compounds which are very useful as 
indicator compounds in an iron assay system. The complex of iron with the 
present compounds not only show a high sensitivity but also a very high 
stability at lower pH-ranges. The stability of the iron-indicator complex 
at lower pH is very desirable, because this allows less drastic conditions 
for the promotion of the release of iron from the transport of the protein 
transferrin. 
The present invention concerns compounds of the general formula (I) 
##STR3## 
wherein R.sup.1 denotes hydrogen, halogen or alkyl and 
R.sup.2 denotes C.sub.1 -C.sub.4 alkyl, aryl or heteroaryl or a substituted 
C.sub.1 -C.sub.4 alkyl, aryl or heteroaryl. 
Preferred are compounds of formula (I) wherein 
R.sup.1 denotes hydrogen and 
R.sup.2 denotes C.sub.1 -C.sub.4 alkyl, aryl or thienyl, which radicals can 
be substituted by C.sub.1 -C.sub.4 alkyl, hydrogen, halogen or SO.sub.3 H. 
More preferred are compounds of formula (I) wherein 
R.sup.1 denotes hydrogen and 
R.sup.2 denotes methyl, ethyl, propyl, phenyl, tolyl or thienyl. 
In particular the present invention concerns compounds of formula (I) 
wherein 
R.sub.1 denotes hydrogen and 
R.sub.2 denotes 
##STR4## 
wherein R.sup.11 denotes hydrogen, methyl or chlorine. 
The most preferred compound is a compound having the formula 
##STR5## 
The present invention is further related to an assay system for detection 
or quantitative determination of iron in a sample comprising one of the 
compounds. 
In a preferred embodiment the iron assay system further comprises a 
reducing compound of the formula 
##STR6## 
Consequently the present invention is also related to the use of this 
compound in the determination of iron. The mentioned quinoline compound 
has been determined as very useful for the reduction of Fe.sup.3+ to 
Fe.sup.2+, which is necessary in the iron test system. Ascorbic acid or 
other reducing agents known in the art can also be used in connection with 
the new indicators of the present invention, but the above quinoline 
compound is preferred for the purpose of reduction. Consequently, the 
present invention is also related to the use of 
3-hydroxy-1,2,3,4-tetra-hydro-benzo(h)-quinoline as reducing agent in an 
assay system for the determination of iron. 
The iron assay-system can furthermore contain substances which do not 
react, such as, for example, buffers, wetting agents, stabilizers and the 
like. 
Reagent combinations can be prepared from the above compounds. The reagent 
combination can be in the form of a solution or as a powder or are in the 
form of tablets or a lyophylisate. The reagent combination (if it is not 
already in the form of a solution) is taken up in water or another 
suitable solvent and a reagent solution is prepared. If the reagent 
combination consists of individual components, these are to be mixed with 
one another. After the sample (for example blood, serum, plasma or urine) 
has been mixed with an aliquot portion of the reagent mixture, the color 
formed is measured on a photometer and the concentration of iron is 
calculated via the molar extinction coefficient and the volumes of reagent 
and sample added, or via an iron standard aqueous solution. 
The iron assay system can furthermore be impregnated, together with a 
buffer system, with appropriate wetting agents and activators as well as 
other auxiliaries, onto absorbent reagent carriers, such as papers, 
fleeces and the like. For this, one or more impregnating solutions can be 
prepared in the form of aqueous, organic or mixed solutions, depending on 
how the reagents or auxiliaries dissolve. Absorbent or swellable carriers, 
preferably filter paper or absorbent fleece of glass or plastic, are 
impregnated or sprayed with these solutions. The carriers are then dried. 
The reagent carriers thus prepared can be used either as rapid diagnostics 
for direct determination of the contents of the analyte in the liquid (for 
example in body fluids, such as blood, urine or saliva, in foodstuffs, for 
example fruit juices, milk and the like). The liquid is thereby applied 
directly to the reagent carrier or this is immersed briefly in the liquid. 
Semiquantitative determination is possible by allocating a comparison 
color to the thus formed. Quantitative evaluation can be carried out by 
reflectance photometry. 
It is also possible to introduce the test agent according to the invention 
into carrier matrices prepared from casting solutions. Examples which my 
be mentioned here are cellulose, cellulose derivatives, gelatin, gelatin 
derivatives or plastics, such as polyurethanes and polyacrylamide. It is 
advantageous here that the test agent and if appropriate the other 
necessary reagents are added directly to the casting solution, which means 
that it is possible to produce the test device, consisting of the carrier 
and reagents, in one operation. 
By eluting the above-mentioned reagents with water or buffer or serum from 
the absorbent carrier, a reagent solution can be prepared, with which the 
analyte or enzymes can be determined in the cell of a photometer as 
described above. 
Wetting agents are, in particular, anionic and cationic, nonionic or 
anphotheric wetting agents. 
Other auxiliaries which my be appropriate are the customary thickeners, 
solubilizing agents, emulsifiers, optical brighteners, contract media and 
the like, such as are known in corresponding tests with other chromogens. 
The preparation of the compounds according to the invention can be 
illustrated by way of an example: 
##STR7## 
The required starting materials are known from the literature [of for 
example: Acta. Chem. Scand., 23, 1087 et seq (1969); J. Amer. Chem. Soc., 
75, 1115 (1953); Organikum, Organisch chemisches Grundpraktikum, page 325 
et seq. (1970)].

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preparative Examples 
Example 1 
6.44 g (grams) of 3-(2-pyridyl)-5,6-bis(2-thienyl)-1,2,4-triazine are 
introduced into 25 ml (milliliter) of 25% strength oleum at 0.degree. C. 
The reaction mixture is allowed to reach room temperature and is stirred 
further for 24 hours. The sulphonation mixture is then discharged on to 
.about.100 g of ice, buffered with a small quantity of NaOH and the 
precipitate is filtered off with suction. 8.4 g of a yellow powder are 
isolated which forms a blue color with iron (II) ions in water 
(.lambda..sub.max =593 nm; c=34000). It is clear from the NMR spectrum 
that the powder is one of the following possible isomers: 
##STR8## 
Example 2 
The 3-(2-pyidyl)-5,6-bis (2-thienyl)-1,2,4-triazine required in Example 1 
is prepared in the following manner: 
13.6 g of picolineamidrazone and 22.2 g of thenil are stirred in 125 ml of 
ethanol at room temperature. After 24 hours the reaction mixture is 
concentrated in a rotary evaporator and the residue is recrystallized from 
absolute ethanol. C - 13 NMR data 
______________________________________ 
##STR9## 
______________________________________ 
C-1 150.40 D 
C-2 125.33 D 
C-3 139.01 D 
C-4 124.04 D 
C-5 160.04 D 
C-6 152.63 S 
C-7 149.54.sup.a S 
C-8 ****.sup.a.S 
C-9 136.89 S 
C-10 131.73 D 
C-11 129.60 D 
C-12 128.83 D 
C-13 137.11 D 
C-14 128.27 D 
C-15 129.81 D 
C-16 132.32 D 
______________________________________ 
Example 3 
284 g of phosphorous pentaoxide are stirred in a mixture of 2500 ml of 
toluene and 250 ml of thiophene. Then 300 g tolylacetic acid are added in 
portions at 80.degree. C. The mixture is subsequently stirred for 5 hours 
and is then discharged onto ice. The organic phase is separated off, dried 
and concentrated in a rotary evaporator. The residue is recrystallized 
from aqueous ethanol. 326 g of the following compound 
##STR10## 
are obtained. 
Example 4 
27.75 g of selenium dioxide are suspended in a mixture of 250 ml of dioxane 
and 20 ml of water. 54.0 g of the thienyl ketone of Example 3 are added to 
this suspension and the mixture is then heated for 6 hours under reflux, 
filtered off by suction to remove the residue and the reaction mixture is 
concentrated in a rotary evaporator. 49.2 g of a brown oil of the 
following formula 
##STR11## 
are obtained and processed further without purification. 
Example 5 
23.0 g of the diketone prepared in Example 4 are heated under reflux with 
13.6 of picolineamidrazone in 100 ml of ethanol. The triazine formed 
already crystallizes out under boiling heat. After 1 hour the mixture is 
cooled and filtered off with suction. 33.9 g of a yellow powder with a 
melting point of 191.degree. C. are isolated. The spectra do not allow the 
product to be assigned definitely to one or other of the following two 
isomers: 
##STR12## 
Example 6 
If the triazine of Example 5 is reacted with 25% strength oleum according 
to the process described in Example 1, the monosulphonated compound is 
obtained which can be assigned to one of the following structures: 
##STR13## 
Calculated: C, 52.77; H, 3.03; N, 12.96; O, 11.1; S, 14.83; N, 5.31. 
Found: C, 52.3; H, 3.1; N, 13.0; S, 15.0. 
Example 7 
If 6.44 g of 3-(2-pyridyl)-5,6-bis(2-thienyl)-1, 2,4-triazine is stirred in 
50 ml of sulphonated monohydrate for 7 hours at 50.degree. C. and the 
mixture is then worked up as described in Example 1, 3.2 g of the 
following compound, which has a melting point of higher than 250.degree. 
C., are obtained: 
##STR14## 
Example 8 
If pyridinylthiophene is reacted with picolineamidrazone as described in 
Example 5 one of the two following possible triazines are obtained in a 
yield of 80% [JR:1385 cm.sup.-1 (-CH.sub.3)]: 
##STR15## 
Example 9 
The triazine derivative of Example 8 is sulphonated in monohydrate at a 
temperature of 50.degree. C. After working up a yellow powder which 
corresponds to one of the following formula, is obtained in a 65% yield. 
##STR16## 
Analysis: 
Calculated: C, 43.82; H, 2.55; N, 15.72; O, 13.47; S, 17.99; Na 6.45. 
Found: H, 44.00; H, 2.45; S, 18.2. 
Test Examples 
In the following iron tests the compound of Example 1 has been used as 
indicator. 
Principle of the Test 
Iron in human serum is released from its carrier protein, transferrin, in 
an acid medium and simultaneously reduced to the ferrous form by a 
reducing agent. Ferrous ions chelate with the indicator forming a stable 
blue complex whose absorbance, spectrophotometrically read at 593 nm 
(nanometers), is proportional to the iron content. Deproteinization is not 
required. A sample blank is required to correct for the serum matrix 
effect. 
MATERIALS AND METHOD 
Experiments (formula optimization, linearity, comparison studies, etc.) 
were carried out according to the following directions: 
Sample: human plasma heparinized or human sera (native or spike with ferric 
ions) obtained from hospital routine were used. Aqueous solutions of iron 
were prepared dissolving from iron metal (NBS material code 937) with 
nitric acid and diluting to the appropriate concentration with distilled 
water. 
Instrumentation: a double-beam spectrophotometer (model Lambda 5, Perkin 
Elmer Corp.) was used. 
Materials: The iron indicator was the compound of Example 1, all other 
compounds were reagent-grade materials. The working solution contains the 
buffer, the reducing agent, thiourea (to suppress a possible copper 
interference) and the indicator compound. A working solution without the 
indicator compound was also prepared for the sample blank tests. For 
comparison studies the SERA-PAK Iron kit, Ferene-S method of Ames 
Division, Miles Italiana S.p.A., was used. 
Test procedure 
______________________________________ 
Wavelength 593 nm (570-610) 
Cuvette 1 cm light path 
Temperature room temperature 
Reading against reagent blank for standard 
and sample; against distilled water 
for sample blank 
______________________________________ 
______________________________________ 
Pipette into test tubes: 
Reagent Sample 
blank blank Standard Sample 
______________________________________ 
Distilled water 
0.20 -- -- -- 
Sample -- 0.20 ml -- 0.20 ml 
Standard -- -- 0.20 ml -- 
Work, Solution 
-- 1.00 ml -- -- 
without indicator 
Work, Solution 
1.00 ml -- 1.00 ml 1.00 ml 
______________________________________ 
Mix and allow to stand at room temperature for 5 min. Read the absorbance 
of the sample blank (Asb) against distilled water and the absorbance of 
the sample (As) and of the standard (Ast) against the reagent blank. 
##EQU1## 
OPTIMIZATION STUDIES 
pH Optimization 
Starting with a formulation containing the following components: 
______________________________________ 
Indicator 3.5 mmol/L 
Thiourea 63 mmol/L 
Ascorbic acid 10 mmol/L 
Buffer 180 mmol/L: pH range 0.5-5.0 
______________________________________ 
The effect of pH on the iron test was studied using 3 aqueous solutions of 
iron at concentrations of approximately 200, 500 and 1000 .mu.g/dl and two 
different human plasma pools at approximately 300 .mu.g/dl of iron. 
Different types of buffer were used to cover the pH range: 
EQU KC1/HC1 for the pH 0.5-1.0-1.5-2.0 
EQU Citric acid/NaOH for the pH 2.0-2.5-3.0 
EQU Acetic acid/HaOH for the pH 3.0-4.5-5.0 
Color development was monitored at 593 nm and the absorbance after 5 min. 
at room temperature (end-point of the reaction) was taken (see Table 1). 
The average absorbance for the three aqueous solutions of iron and for the 
two pools of plasma were calculated and plotted vs. pH (FIG. 1) 
From the data it is evident that the indicator compound according to the 
invention can be used for pH values equal or greater than 1, and 
preferably of about 1 to facilitate the release of iron from transferrin. 
Choice of the buffer 
Compounds able to give buffer solution at pH 1 were selected; e.g. using 
the KC1/HC1 or citric acid or malonic acid as buffer agents and testing at 
pH 1.0 and 0.3 mol/L aqueous solution of iron and human plasma. No 
difference in absorbance response and time of reaction were noticed. All 
the compounds tested were found to have the same buffer capacity with 
human sera. 
Choice and Optimization of the Reducing Agent 
Ascorbic acid is the reducing agent generally used to reduce ferric ions; 
unfortunately the compound is stable only for a few hours when it is put 
into solution. Consequently, in general the iron kits commercially 
available, supply the ascorbic acid in powder form to be added manually to 
a preformed solution (i.e. see Sera-Pak Iron kit). In order to obtain a 
ready to use solution a search was carried out to find a more stable and 
appropriate reducing agent. The 3-hydroxy-1,2,3,4-tetrahydro-benzo(h) 
quinoline (HTBQ) 
##STR17## 
was found very suitable, in an acid medium, the ascorbic acid to reduce 
the ferric ions to ferrous ions and to promote the iron release from 
transferrin. 
Starting with a formulation containing the following components: 
______________________________________ 
HCl/KCl buffer pH 1.0; 
100 mmol/L 
Indicator 3.5 mmol/L 
Thiourea 63 mmol/L 
______________________________________ 
The HTBQ was added in concentrations ranging from 0 to 25 mmol/L and the 
absorbance response to 593 nm after 5 min of reaction was recorded using 
aqueous solutions of iron and two different human plasma pools. Data in 
FIG. 2 show that a minimum amount of 5-10 mmol/L of HTBQ is required; a 
concentration of about 10 mmol/L is preferable. 
Indicator Optimization 
Starting with a formulation containing: 
______________________________________ 
HCl/KCl buffer pH 1.0; 
200 mmol/L 
Thiourea 63 mmol/L 
HTBQ 20 mmol/L 
______________________________________ 
The indicator was added in concentrations ranging from 0.5 to 10 mmol/L and 
the absorbance response at 593 nm after 5 min. of reaction was monitored 
using aqueous solutions of iron and two different pools of human plasma. 
Data in FIG. 3 show that a minimum amount of 2.5-3 mmol/L of the indicator 
is required; a concentration of about 3.5 mmol/L is preferable. 
From the optimization studies carried out the following remarks can be 
made: 
1. pH: the system works in the pH range from 1 to 5; a pH=1 is preferred to 
facilitate the dissociation of iron from transferrin. Choosing a pH over 
3.0 it is preferable to introduce in the working solution a surfactant to 
avoid possible sample turbidity; Triton X-100 or Tween 10 at a 
concentration of 0.5% can be used. pH higher than 5 were not tested but it 
is presumable that the system could work if an appropriate component to 
dissociate iron from transferrin is used. 
2. Buffer: different types of compounds can be used (e.g. citric acid, 
malonic acid, HC1/KC1). 
3. Molarity; a buffer molarity less than 400 mmol/L and preferably of about 
200 mmol/L is preferred to avoid possible human sample turbidity; however 
a molarity higher than 400 mmol/L can be used in appropriate surfactants 
are used. 
4. Reducing Agent: HTBQ can conveniently substitute the ascorbic acid. A 
concentration above 5 mmol/L is suggested, a concentration of about 20 
mmol/L is preferred. 
5. Indicator Compound: a concentration above 2.5 mmol/L is suggested, a 
concentration of about 3,5 mmol/L is preferred. 
6. Thiourea: using a formulation at pH 1.0 this component is not necessary 
and its use can be avoided; above this pH value a concentration of 63 
mmol/L is satisfactory to suppress copper interference, (data not shown). 
PERFORMANCE VALIDATION 
The performance validation was carried out according to the test procedure 
initially reported and with formulation containing the following 
components: 
______________________________________ 
HCl/KCl Buffer pH 1.0; 
200 mmol/L 
Indicator 3.5 mmol/L 
HTBQ 20 mmol/L 
______________________________________ 
Linearity tests 
Aqueous standards of ferric ions, prepared by dissolving iron metal (NBS 
material) in nitric acid and diluted to appropriate concentrations with 
distilled water, were assayed in triplicate. FIG. 4 shows a linearity up 
to at least 1000 .mu.g/dl iron. 
Comparison study 
Comparative assays were conducted using the Sera-Pak Iron kit and the 
present formulation. 25 human plasma heparin were used as samples. 
The results obtained, elaborated statistically by a linear method, are 
shown in FIG. 5. The correlation between the two methods is very good.