Depends on how you define conspiracy. It takes about a billion dollars to do all the clinical trials to bring a drug to market. And pharma usually picks up the tab. Some substances that are low cost are effective for cancer, however if they ever came to market pharma's $100,000 a cycle chemotherapy agents would have to compete with agents that cost $50 a cycle. So no way in hell Pharma is funding them.
So nobody can get the funding to do the trials. Private investment and public investment helps, but its not enough. Yet another danger of living in a plutocracy. Its not so much that pharma blocks new agents, just that they control all the major purse strings and hold tons of sway with 'medical professionals' so they can influence their decision making and can starve out cheaper, more efficient alternatives. How many doctors recommend magnesium citrate and vitamin D for depression? Those work well, but they don't make money for pharma.
Substances that inhibit glycolysis like dichloroacetic acid (DCA) and 3-bromopyruvate are dirt cheap, and because (I think all forms of) cancer uses glycolysis for energy, these drugs help kill cancer cells while leaving healthy cells alive. And DCA is already used to treat lactic acidosis, so it isn't a foreign substance that we don't understand. It is already used in medicine.
However DCA and 3-bromopyruvate cannot be patented. So anyone can make them. And a months supply likely only runs $20, vs the 10s of thousands you'd spend on pharma chemotherapy agents.
http://www.thedcasite.com/http://www.hopkinsmedicine.org/press/2002/July/020715.htmhttp://toxipedia.org/display/toxipedia/Dichloroacetic+Acid#DichloroaceticAcid-UseAgainstCancerThe solution is to have more public funding of medical research, and to lower the costs of bringing a drug to market.
I believe pharma is working on drugs that also inhibit glycolysis, but those drugs will be patent protected. DCA has been used for years as a treatment for lactic acidosis, so we know it is safe for human consumption. But nobody can make money on it. So it has basically been buried as a cancer treatment.
When Pharma does bring glycolysis inhibitors to market, they will cost 100x+ more than DCA, and possibly have worse side effects (due to being on the market for less time).
Either way, again, all a problem of Reaganomics and letting a handful of powerful individuals and corporations control too many levers of power.