When you are considering cancer, you are certainly considering a change
in genotype and phenotype of particular cells within the individual.
Cancer is almost uniformly due to changes in DNA, though there are a
huge variety of reasons that this can happen, hence a huge variety of
treatments are applicable. With cancer, deletions of regulatory proteins
(receptors, secondary messenger regulators...) can result in normal
messages going haywire. If any combination of damage messes with the
proliferation signalling pathways, then all of the sudden, cells can
begin to rapidly divide with no way of messenging them to stop.

Resistance to drugs happens through evolution, and is a function of
diversity in the gene pool (all the human individuals), or through
mutation of genes to confer resistance. If some common drug was suddenly
discovered to cause sterility in most of the gene pool, the few
individuals with natural resistance (lets say 0.1% of the population)
would still reproduce, and the following generation would all be
resistant to the sterility effect.

drug effects are often dramatic, but don't confuse a physiological
response to a drug with the indiscriminate damage that env. insults,
smoking, chem exposure result in.

Drug effects are usually dramatic because they change normal messages in
the body for a periof of time, and are metabolized or excreted by the
body, at which point they can no longer have an effect on the body.

The other insults may be slow, but the damage accumulates over time.
For example, UV radiation hits your body  from sunlight. Lets say the
radiation causes a base to be copied incorrectly in one cell, resulting
in an early stop codon in the gene which codes for insulin. Every time
this cell and its progenitors divide, the wrecked version of insulin is
propogated.

If this happens anywhere in your body other than the islets of
langerhans, (the cells which make insulin) or your germline cells
(sperm/egg production), the damage doesn't do anything.

So it's entirely possible that your skin cells have undergone thousands
of mutations already, but the mutations are not important to the
function and reproduction of skin cells.

Eventually, most people who expose themselves to massive doses of
sunlight get skin cancer. At some point, mutations which affect the
reproduction of skin cells accumulate to the point that the cells stop
responding to the normal growth signals, and proliferate wildly,
resulting in ugly tumours.

The genetic damage stays, but the drug exposure is time limited.

To get an idea of which drugs can be antigenic, and why they can go look
up penicillin, and hapten. After that, consider what introducing cow
insulin into the human body may do.

After that, imagine insulin was massproduced by bacteria, and then
injected into humans, would there be a larger immune response to the
bacterial produced insulin as compared to the cow insulin?

Most drugs are not antigenic!

Because cancer is essentially loss-of-function, whereas resistance to a
drug is gain-of-function.  When you get cancer what happens is the genes
which regulate cell division no longer work properly.  To develop
resistance you need to gain new genes (or at a minimum, alter an old
one) to get resistance.  As you can imagine, it is much easier to break
something that already exists then it is to create something new.

That's not to say you couldn't have a cell develop resistance to a drug,
but:

1) How does having one (or a few) cells which are resistant prevent
toxicity in the trillions of cells in your body?
A) it doesn't.

2) How does that get passed on to your kids?
A) It doesn't, unless the mutation happened to occur in an egg/sperm.

Phenotype generally follows genotype, so it's pretty hard to have large
changes in phenotype (i.e. development of resistance) without a
concurrent change in genotype.

You have to be very careful when talking about the cause of cancer.  In
all cases, cancer is a result of uncontrolled cell division.  AFAIK, in
100% of cases this is a result of mutation(s) in the genes which
regulate cell division.

However, what causes these mutation(s) is not well understood.  We know
that a lot of cancer is linked to infectious disease (notably viral and
intracellular bacteria).  Others are due to environmental pollutants
(smoking), radiation (skin cancer), genetics (reteroblastoma), etc.
Some seems to be just plain bad luck.  But it is pretty hard to take a
single patient and say that exposure to X caused their cancer.

Drug can, but all drugs go through a thorough screening process called a
"clinical trial".  This takes 5-10 years, and has two major goals:

1) To determine if the drug has medical benefit, and
2) To determine if the drug has side effects.

Cancer is a potential side effect of a drug, but such a drug would never
be allowed to be sold.  This is why we have clinical trials - to weed
out drugs with really bad side effects (like cancer) BEFORE the drug
reaches market.

You also have to keep in mind that cancer is, in essence, a DNA-based
disease.  So a drug would have to effect DNA, or the DNA repair enzymes,
to have the potential to cause cancer.  Most drugs to not target DNA or
repair enzymes, and as such don't really have the potential to cause cancer.

Bryan

Do you know if insulin is excreted in urine or not? It is accepted that
on getting diabetic nephropathy, insulin is excreeted lesser. So why
not in normal people?
Mike McWilliams ne likha tha: