Keywords Lipid peroxidation - oral cancer detection - oral cancer diagnosis - oral cancer in
chronic smokers - salivary diagnostic test for oral cancer
Introduction
High mortality in oral cancer attributed mainly to late diagnosis
Oral cancer also known as mouth cancer is any cancerous tissue growth located in the
oral cavity.[1 ] It most commonly involves the tongue, floor of the mouth, cheek lining, gums, lips,
or roof of the mouth. More than 90% of all oral cancers are squamous cell carcinoma.[2 ]
By country, the incidence of oral cancer is the highest in India, which accounts for
almost one-third of cases found in the world.[1 ] Over five people in India die every hour because of oral cancer.[3 ] Public health centers and private hospitals have recognized oral cancer as a grave
problem and efforts toward early detection and prevention can help reduce this burden.
The high prevalence of oral cancer in India is mainly due to the influence of tobacco
and betel quid chewing.[4 ] Greater than 90% of patients with oral cancer report using tobacco products.[5 ] The incidence of oral cancer in patients who smoke and chew tobacco is 8.4 times
greater than that of patients who do not.[6 ]
Globally, the 5-year mortality rate of oral cancer is approximately 50% and has not
improved despite advances in diagnostic techniques and improvements in treatment modalities.[4 ] The high mortality rate in oral cancer is attributed to late diagnosis, which is
either due to lack of knowledge or access to medical care.[1 ],[2 ],[7 ] Most patients seek help only in later stages when symptoms such as pain, ulceration,
or a neck mass appear.[1 ]
Detection of an oral cancer at Stage I carries a prognosis of 80% survival, while
the same lesion at Stage III carries a prognosis of 20% survival.[8 ] This difference could affect not only the quality of life for the patients, but
also the cost of the medical treatment. Thus, there is a need for improvement in early
risk detection of oral carcinomas, because in the initial and precancerous stages,
treatment is more effective and morbidity is minimal.
Link between free radicals, lipid peroxidation, and oral precancer and cancer
Free radical-induced lipid peroxidation (LP) has been implicated in the pathogenesis
of oral cancer.[9 ],[10 ],[11 ],[12 ],[13 ] Smoking generates free radicals and reactive oxygen species (ROS). ROS-induced cell
damage causes LP. It most commonly affects polyunsaturated fatty acids, which causes
alteration in the structure and function of cell membranes and also initiates and
promotes the multistep process of carcinogenesis.[10 ]
Malondialdehyde (MDA) is the end product of LP and can be used as a marker for assessing
the extent of LP.[9 ],[10 ],[11 ],[12 ],[13 ] MDA is mutagenic and genotoxic, as it readily reacts with deoxynucleosides to produce
adducts that cause DNA damage.[11 ] An increase in MDA concentrations in saliva has been widely reported in various
oral precancers and cancers in the early stages.[9 ],[10 ],[11 ],[12 ],[13 ] The salivary MDA in potentially malignant diseases is found to be in the range of
25–50 ng/ml and that in oral squamous cell carcinoma in the range of 50–100 ng/ml.[9 ],[11 ],[14 ]
In addition, previous studies have shown that salivary MDA could serve as a potential
diagnostic marker, and that by measuring the salivary MDA level, we can measure the
extent of LP and thereby assess the risk of a smoker toward developing oral precancer
and oral cancer.[9 ],[10 ],[12 ],[13 ],[14 ],[15 ]
With this background, we were motivated to investigate the relationship between smoking,
LP, and oral cancer and hence we carried out our study. We hypothesized that the heavier
the smoker, the greater the level of salivary LP and the higher the risk of developing
oral precancer and cancer. To test our hypothesis, we devised a simple, quick, colorimetric,
home-based test named QuitPuff to determine the salivary MDA levels in smokers.
Materials and Methods
In this study, a simple home-based diagnostic method named QuitPuff was developed
for the determination of LP in saliva by simple means, i.e., through a visible color
change which can be observed by naked eye.
2-Thiobarbituric acid (TBA) 98% was purchased from Sigma-Aldrich; MDA tetrabutylammonium
salt 96% pure was purchased from Sigma-Aldrich. Extra pure distilled water was used.
Trichloroacetic acid 1% and orthophosphoric acid 85% pure were purchased from Emplura.
The study was conducted on patients from two hospitals and a clinic. The sample analysis
and experiments were conducted at a regulated research institution.
Preparation of malondialdehyde standards in saliva of healthy nonsmokers
As our diagnostic medium was saliva, the MDA standards were also prepared in saliva.
Ten healthy subjects were selected in the age group of 30–45 years, with no prior
history of smoking or tobacco chewing. To rule out any preexisting MDA, their saliva
samples were sent to a regulated institution for MDA detection through liquid chromatography–mass
spectroscopy (LCMS). The LCMS report confirmed the absence of MDA in all 10 samples
collected from the healthy people.
10 mL of unstimulated saliva was collected from each of the ten healthy subjects.
MDA standards using saliva of each of the 10 healthy subjects were prepared in the
concentrations of 500 ng/mL, 250 ng/mL, 100 ng/mL, 50 ng/mL, 25 ng/mL, and 5 ng/mL.
Preparation of the diagnostic QuitPuff reagent
One molecule of MDA reacts with two molecules of TBA under high temperature and acidic
conditions to produce a colored MDA-TBA adduct. This is the basic principle of TBARS
assay which is the most widely employed assay used to determine LP. Although being
accurate, these tests are not routinely done in laboratories. They are expensive,
require technical skills, laboratories, and instrumentation, and sometimes need complex
methods such as high-performance liquid chromatography or LCMS. Thus, they are fairly
out of reach of ordinary people.
As the aim of our study was to develop a simple, home-based test, we explored the
possibility of converting a complex laboratory-based TBARS assay into a simple colorimetric
test that could be self-conducted by the user and the results could be interpreted
by simply matching the color change in a sample of saliva to a color chart.
To formulate the most sensitive TBA reagent that would detect MDA concentrations as
low as 5 ng/mL, by producing a visible color change, MDA standards using saliva of
10 healthy subjects were prepared in the concentrations of 500 ng/mL, 250 ng/mL, 100
ng/mL, 50 ng/mL, 25 ng/mL and 5 ng/mL and TBA reaction was performed using various
TBA formulations that were adapted from previous studies.[16 ]-[20 ]
[Table 1 ].
Table 1
Derivation of the QuitPuff reagent
Method no.
TBA formulation
TBA reaction
Detection limit of color change
Adaptability as a point-of-care
Method 1
Formulation adapted from Ohkawa et al.[16 ], 0.2mL of 8.1% sodium dodecyl sulphate (SDS), 1.5mL 20% acetic acid at pH3.5, 1.5mL
of 0.8% of Thiobarbituric Acid (TBA)
0.2 mL TBA reagent was added to 0.2 ml of each of the MDA standards as well as a saliva
control and boiled in water bath for 1 hour at 95 degree Celsius.
100 ng/mL
#
Method 2
Formulation adapted from Buege J.A and S.D. Aust et al.[17 ], 0.5% Thiobarbituric Acid, 20% Trichloro Acetic acid, 2.5N Hydrochloric acid Formulation
adapted from
0.5 mL TBA reagent was added to 0.5 mL of each of the MDA standards as well as a saliva
control and heated in a boiling water bath for 20 minutes.
100 ng/mL
#
Method 3
Esterbauer H. et al.[18 ], 1 mL 0.37% Ethylenediaminetetraacetic-acid (EDTA), 1 mL 2% Butylated hydroxytoluene
(BHT), 2 mL 10% Trichloro Acetic Acid, 1 ml 0.67% Thiobarbituric Acid.
5 ml TBA reagent was added to 1ml of each of the MDA standards as well as a saliva
control and heated in a boiling water bath for 10 minutes.
100 ng/mL
#
Method 4
Formulation adapted from Uchiyama M and Midori Mihara M. [19 ], 3 mL 1% Phosohoric acid, 1 mL 0.6% Thiobarbituric acid aqueous solution
TBA reagent was added to 0.5 mL of each of the MDA standards as well as a saliva control
and heated in a boiling water bath for 45 minutes.
25 ng/mL
#
Method 5
Formulation adapted from Asakawa T. and Matsushita S.[20 ], 2 mL of 20% Trichloro Acetic acid containing 20 umol Ferrous Sulphate, 1 mL 0.67%
Thiobarbituric acid
TBA reagent was added to 1 mL of each of the MDA standards as well as a saliva control
and heated in a boiling water bath for 30 minutes.
25 ng/mL
#
Method 6
Formulation adapted from experimentation 0.1875 g of Thiobarbituric Acid in 0.4562
mL of Acetic Acid, added distilled water to bring the volume to 50 mL
2 mL TBA reagent was added to 1 mL of each of the MDA standards as well as a saliva
control and heated in a boiling water bath for 15 minutes.
5 ng/mL, color change observed in control
#
Method 7
Formulation adapted from experimentation. 1 mL 0.375 g of Thiobarbituric Acid in 1
mL 85% Ortho-Phosphoric Acid.
2 mL TBA reagent was added to 1 mL of each of the MDA standards as well as a saliva
control and heated in a boiling water bath for 15 minutes.
25 ng/mL
#
Method 8
Formulation adapted from experimentation 0.375 g of Thiobarbituric Acid in 1 mL 85%
Ortho-Phosphoric Acid, 1 mL 1% Trichloro Acetic Acid.
2 mL TBA reagent was added to 1 mL of each of the MDA standards as well as a saliva
control and heated in a boiling water bath for 15 minutes.
5 ng/mL
@
Method 9
Formulation adapted from experimentation 0.375 g of TBA in 1 mL 1% Ascorbic Acid,
1 mL 1% Trichloro Acetic Acid.
2 mL TBA reagent was added to 1 mL of each of the MDA standards as well as a saliva
control and heated in a boiling water bath for 15 minutes.
5 ng/mL, color change observed in control
#
Method 10
Formulation adapted from experimentation 0.3 g of TBA dissolved in 3 mL of Dimethyl
Sulfoxide (DMSO).
2 mL TBA reagent was added to 1 mL of each of the MDA standards as well as a saliva
control and heated in a boiling water bath for 15 minutes.
25 ng/mL
#
As shown in [Table 1 ], the formulation used in method 8 proved to be most sensitive, detecting MDA levels
as low as 5 ng/mL by producing a visible color change. Thus, the QuitPuff TBA Reagent
was prepared by dissolving 0.375 g of TBA in 85% orthophosphoric acid (1 ml) and 1%
trichloroacetic acid (1 ml).
Thiobarbituric acid – Reaction on known standards using QuitPuff
Using the QuitPuff diagnostic TBA reagent, TBA reaction was carried out on the known
standards of MDA to generate a visibly colored MDA-TBA adduct. 2 mL of TBA reagent
was added to 1 mL of each of the six MDA standards as well as a saliva control. The
mixtures were then heated in a boiling water bath for 15 min and the color change
was observed. The color change was observed in a gradient, with A being the darkest
and F being the lightest. The color of control in the saliva control remained unchanged
[Figure 1]a ].
Figure 1: (a) Thiobarbituric acid reaction on malondialdehyde saliva standards of
healthy nonsmokers. The color change is observed in a gradient with a being the darkest
and F being the lightest. No color change is seen in the control. (b) The standard
curve, plotted based on optical density and malondialdehyde concentration
The colored samples were analyzed through ultraviolet (UV) visible spectroscopy and
the absorbances were measured at 532 nm. Based on the readings, a standard curve was
prepared [Figure 1]b ]. The optical density was plotted along the Y-axis and the MDA concentration in ng/mL
was plotted along the X-axis. The following linear equation was derived: (Y = 0.0049X
+ 0.1472).
Preparation of colorimetric chart or the lipid peroxidation index
Based on the color change and UV spectroscopy readings, a colorimetric chart was prepared
[Figure 2 ]. The colors are numbered from zero to six and denote the LP index (LPI). The results
can be interpreted by noting the LPI, which indicates the degree of LP, and thereby
the risk status.
Figure 2: The colorimetric LPI chart. The numbers from 0 to 6 denote the LPI. Interpretation
of Results: LPI 0 denotes-no risk; LPI 1 denotes-low risk; LPI 2–3 denotes-moderate
risk; LPI 4–6 denotes-high risk. LPI – Lipid peroxidation index
Testing the method on 500 subjects: The QuitPuff saliva kit was tested on 500 subjects
(aged 30–60 years) from two hospitals and a clinic. These subjects were divided into
5 groups. Each group consisted of 100 subjects. Informed consent was obtained from
all groups.
Inclusion criteria
The etiology of oral cancer in India is dominated mainly by tobacco use, alcohol consumption,
and smoking.[3 ],[4 ],[5 ],[6 ],[7 ] As the aim of our study was to investigate the relationship between smoking and
oral cancer, subjects with significant alcohol consumption and tobacco chewing habits
were excluded.
Nonsmokers (n = 100) – No history of alcohol consumption or tobacco chewing habit,
average tar content inhaled per day by each subject is 0 mg, no comorbidity, and no
oral lesions.
Smokers who smoked <10 cigarettes per day for a minimum of 5 years (n = 100) – No
significant alcohol consumption or tobacco chewing habit, average tar content inhaled
per day by each subject is 120 mg, no comorbidity, and no oral lesions.
Smokers who smoked 10–20 cigarettes per day for a minimum of 5 years (n = 100) – No
significant alcohol consumption or tobacco chewing habit, average tar content inhaled
per day by each subject is 180 mg, no comorbidity, and no oral lesions.
Smokers who smoked above 20 cigarettes per day for a minimum of 5 years (n = 100)
– No significant alcohol consumption or tobacco chewing habit, average tar content
inhaled per day by each subject is 240 mg, no comorbidity, and no oral lesions.
Smokers who smoked 10–20 cigarettes per day for a minimum of 5 years with recently
diagnosed precancerous mouth lesions and oral cancer Stage 1–2 yet to start on treatment
(n = 100) – No significant alcohol consumption or tobacco chewing habit, average tar
content inhaled per day by each subject is 180 mg, oral lesions present, oral erosive
lichen planus, n = 48; oral leukoplakia (raised, indurated, white lesion in the oral
mucosa more than 5 mm in diameter with dysplastic changes in the epithelium), n =
24; submucous fibrosis, n = 8; and oral squamous cell carcinoma (OSCC), n = 20. Of
the twenty subjects with OSCC, sixteen were diagnosed with Stage 1 OSCC and four were
diagnosed with Stage 2 OSCC.
Exclusion criteria
Patients with chronic alcohol addiction, tobacco chewing habits, and patients with
advanced stages of oral squamous cell carcinoma and/or already on treatment or operated
upon were excluded from the study.
The tests were performed on fresh samples of saliva. Subjects were asked not to eat,
drink, smoke, or chew tobacco an hour before the collection. Subjects rinsed their
mouths with 10 mL of water and thereafter saliva was collected in sterile bottles.
The QuitPuff TBA reaction was performed as previously described. The color change
was matched with the colorimetric chart [Figure 2 ] and the LPI was noted.
Validation by ultraviolet visible spectroscopy, the gold standard for the test
For further validation, all 500 samples were sent for MDA determination through UV
Spectroscopy (Supporting Information). The readings of UV visible spectroscopy were
then plotted on the standard curve and the linear equation (Y = 0.0049X + 0.1472)
was used to determine the MDA concentration values. A Spearman's correlation analysis
was performed to understand the correlation between the color change in the samples
and the readings of UV visible spectroscopy. On the basis of the MDA concentration,
the LPI was again derived and noted. A detailed report of these 500 samples, including
photos, UV spectroscopy results, MDA concentrations, and LPI classification, is available
in this article's Supporting Information.
Statistical analysis used
A Spearman's correlation analysis was performed to understand the correlation between
the color change in the samples produced by the QuitPuff reaction and the readings
of UV visible spectroscopy. A Student's t-test was performed to determine the difference
in the mean LPI in each of the smoker's group as compared to the nonsmoker's group.
Results
The mean LPI of nonsmokers was 0.26. In the smoker's group, the mean LPI of smokers
who smoked <10 cigarettes a day was 2.43, that of smokers who smoked 10–20 cigarettes
a day was 3.43, and that of smokers who smoked more than 20 cigarettes a day was 4.12.
The mean LPI of smokers with oral precancer and oral squamous cell carcinoma was 4.34
[Table 2 ].
Table 2
Comparison of mean lipid peroxidation indexes between two methods
Group name
n
Mean LPI derived by colorimetric method
Mean LPI derived by validation method of UV Spec
Non-smokers
100
0.26 ± 0.52
0.23 ± 0.50
Smokers <10 cigarettes/day
100
2.43 ± 1.60
2.46 ± 1.66
Smokers 10-20 cigarettes/day
100
3.43 ± 1.73
3.44 ± 1.75
Smokers >20 cigarettes/day
100
4.12 ± 1.53
4.18 ± 1.55
Smokers with recent oral precancer & cancer lesions (stage 1-2)
100
4.34 ± 0.74
4.37 ± 0.69
The mean LPI was consistently and significantly elevated (P< 0.001) in smokers with
oral precancer and cancer and smokers who smoked more than 20 cigarettes a day, 10–20
cigarettes a day, and <10 cigarettes a day as compared to nonsmokers.
The mean LPI obtained by QuitPuff colorimetric method was compared with the mean LPI
obtained by the validation method of UV spectroscopy. The mean LPIs from both methods
were in agreement [Table 2 ]. Results of the Spearman's correlation indicated that there was a significant positive
association between the color changes obtained by QuitPuff method and the readings
of the UV spectroscopy (r = 0.93, P < 0.001).
Among smokers, 96 of 100 smokers with oral precancer and cancer were found to have
high degree of LP, an increase compared to that of otherwise healthy smokers (83/100
subjects who smoked more than 20 cigarettes a day, 65/100 who smoked 10–20 cigarettes
a day, and 35/100 who smoked <10 cigarettes a day). In contrast, in the nonsmoker's
group, 82 of 100 subjects (82%) had no detectable LP, and none showed a high degree
of LP [Table 3 ].
Table 3
Degree of lipid peroxidation in the study groups
Group name
n
Zero
Low
Moderate
High
Non-smokers
100
82
18
4
0
Smokers <10 cigarettes/day
100
22
9
34
35
Smokers 10-20 cigarettes/day
100
12
5
18
65
Smokers >20 cigarettes/day
100
9
2
6
83
Smokers with recent oral precancer & cancer lesions (stage 1-2)
100
0
0
4
96
Smokers exhibited greater degree of salivary LP as compared to nonsmokers and heavier
the smoker, greater was the degree of salivary LP [Table 3 ].
Discussion
Cancer is caused by the accumulation of multiple lesions occurring in a single cell.
It can be described by three stages: initiation, promotion, and progression. Previous
reports indicate that ROS not only initiate but also promote multistep carcinogenesis.[10 ] ROS-induced LP is implicated in the pathogenesis of oral cancer.[9 ],[10 ],[11 ],[12 ],[13 ] MDA is the end product and the most widely studied product of LP.[9 ],[10 ],[11 ],[12 ],[13 ] By measuring the level of salivary MDA, we can determine the degree of salivary
LP and thereby assess the risk of a smoker developing oral precancer and oral cancer.
In our study, smokers with oral precancer and oral squamous cell carcinoma were found
to have a higher degree of salivary LP as compared to nonsmokers. Within the smoker's
category, the heavier the smoker, the greater the degree of salivary LP. This is in
line with other studies,[9 ],[10 ],[11 ],[12 ],[13 ] on oral precancer and cancer, which have reported similar findings, validating the
relationship between free radical activity, LP, and cancer.
We have also observed in our study that saliva can be used as a suitable diagnostic
medium, as its collection is easy, noninvasive, not time-consuming, and inexpensive.
Our findings are thus in accordance with studies that have shown that salivary MDA
could serve as a potential diagnostic marker in potentially malignant disease and
OSCC.[9 ],[10 ],[12 ],[13 ],[14 ],[15 ] Our diagnostic test was able to detect the salivary MDA level as an indicator of
the degree of LP in the saliva of smokers, precancerous mouth lesions, and OSCC patients
with 97% accuracy, and therefore could serve as an early, safe, noninvasive test for
smokers to assess their risk of developing oral precancer and cancer.
The simplicity of the procedure does not rule out scope for systematic errors. Inappropriate
saliva collection, contamination, or possible degradation of saliva after collection,
as well as errors in measurement of saliva or reagent and inconsistent temperatures
of the water bath, could lead to false positive results. One way to overcome these
errors would be to repeat the test on different occasions. Furthermore, as the test
is colorimetric, interpretation of results is largely subjective. In application,
QuitPuff salivary diagnostic test does not aim to substitute conventional diagnostic
tests, but is meant to act as an qualitative, self-diagnostic test for flagging early
risk, and to be followed by laboratory tests for confirmation of diagnosis.
There is ample awareness that cigarette smoking can lead to cancer. For years, cigarette
packets have been portraying images of cancer lesions, but this has not discouraged
people from smoking. The number of smokers in India has risen from 79 million in 1998
to 108 million in 2015.[21 ] As the oral cavity is more accessible to complete examination, it could be used
in early detection of oral precancerous and cancerous lesions. The disease however
gets detected in the later stages, due to reasons such as lack of knowledge and inaccessibility
of medical care.[7 ] Careful annual examination of the oral cavity in persons above the age of 40 years
can result in significant improvement in the rate of early detection of oral cancer
with all the therapeutic advantages. However, a great proportion of those at risk
of oral cancer do not attend annual health-care checkups.[4 ]
Our aim was to devise a test that could detect the early risk of oral precancer and
cancer in smokers. We propose that if such a simple, do-it-yourself, home-based test
could be provided with every cigarette pack, more people might be willing to check
their risk and make behavioral changes prior to the development of oral cancer. More
people taking these tests would mean more people finding out their risks in the early
stages. Early detection could enable better treatment outcome and improvement in the
quality of life. It could reduce health-care costs and the economic burden of treating
oral cancer. The test kit requires no elaborate storage conditions and could be easily
transported to remote locations and stored in small pan-beedi shops (tobacco selling
shops and kiosks) in rural areas. At per sample cost of Rs. 38.15, it could offer
an inexpensive and affordable option especially to the lower income populations, where
such a test is often most needed. On account of its simplicity, noninvasive nature,
low-cost, and easy accessibility, QuitPuff may have great potential as a point-of-care
test for oral precancer and oral cancer. The test could be useful as a mass screening
tool not only for routine clinics, but also for rural areas and remote locations with
limited laboratory facilities or minimally trained health workers.
Conclusion
On comparing results from the both methods, i.e., the QuitPuff colorimetric method
and the validation method of UV spectroscopy, it was found that only 20 of 500 (4%)
resulted in misclassification error. Thus, QuitPuff was able to detect the degree
of salivary LP with 96% accuracy. QuitPuff has great potential as a mass screening
tool for early risk detection of oral precancer and cancer in smokers, to be followed
up with confirmatory laboratory tests such as biopsies and MRI and CT Scans.
